BackboneJS Front-End Web Apps with Parse Cloud Storage Tutorial Part 4: Managing Unidirectional Associations

Learn how to manage unidirectional associations between object types, such as the associations assigning publishers and authors to books, using BackboneJS in combination with the cloud storage service Parse.com

Gerd Wagner

Warning: This tutorial manuscript may still contain errors and may still be incomplete in certain respects. Please report any issue to Gerd Wagner at G.Wagner@b-tu.de.

This tutorial is also available in the following formats: PDF. See also the project page, or run the example app from our server, or download it as a ZIP archive file.

This tutorial article, along with any associated source code, is licensed under The Code Project Open License (CPOL), implying that the associated code is provided "as-is", can be modified to create derivative works, can be redistributed, and can be used in commercial applications, but the article must not be distributed or republished without the author's consent.

2015-06-14

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Table of Contents

Foreword
1. Reference Properties and Unidirectional Associations
1. References and Reference Properties
2. Referential Integrity
3. Modeling Reference Properties as Unidirectional Associations
4. Representing Unidirectional Associations as Reference Properties
5. Adding Directionality to a Non-Directed Association
6. Our Running Example
7. Eliminating Unidirectional Associations
7.1. The basic elimination procedure restricted to unidirectional associations
7.2. Eliminating associations from the Publisher-Book-Author design model
8. Rendering Reference Properties in the User Interface
2. Implementing Unidirectional Functional Associations with BackboneJS/Parse
1. Handling Single-Valued Reference Properties with BackboneJS/Parse
2. Make a BackboneJS/Parse Data Model
3. New issues
4. Write the Model Code
4.1. Summary
4.2. Encode each model class as an extension of Parse.Object
4.3. Encode the property checks
4.4. Implement a deletion policy
5. The View and Controller Layers
5.1. Initialize the app
5.2. Show information about associated objects in the List Objects use case
5.3. Allow selecting associated objects in the create and update use cases
3. Implementing Unidirectional Non-Functional Associations with BackboneJS/Parse
1. Handling Multi-Valued Reference Properties with BackboneJS/Parse
2. Make a Parse/BackboneJS Data Model
3. New issues
4. Write the Model Code
5. Write the User Interface Code
5.1. Show information about associated objects in the List Objects use case
5.2. Allow selecting associated objects in the create use case
5.3. Allow selecting associated objects in the update use case
6. Run the App and Get the Code
7. Points of Attention

List of Figures

1.1. A committee has a club member as chair expressed by the reference property chair
1.2. A committee has a club member as chair expressed by an association end with a "dot"
1.3. Representing the unidirectional association ClubMember has Committee as chairedCommittee as a reference property
1.4. A model of the Committee-has-ClubMember-as-chair association without ownership dots
1.5. Modeling a bidirectional association between Committee and ClubMember
1.6. The Publisher-Book information design model with a unidirectional association
1.7. The Publisher-Book-Author information design model with two unidirectional associations
1.8. Turn a non-functional target association end into a corresponding reference property
1.9. The association-free Publisher-Book design model
1.10. The association-free Publisher-Book-Author design model
2.1. The complete BackboneJS/Parse data model

List of Tables

1.1. An example of an association table
1.2. Different terminologies
1.3. Functionality types

Foreword

This tutorial is Part 4 of our series of six tutorials about engineering a frontend web application with BackboneJS/Parse. It shows how to build a web app that takes care of the three object types Publisher, Book and Author, as well as of the unidirectional associations that assign authors and a publisher to a book.

The app supports the four standard data management operations (Create/Read/Update/Delete). It extends the example app of parts 2 and 3 by adding code for handling unidirectional associations, but it needs to be enhanced by adding further important parts of the app's overall functionality:

  • Part 4: Managing bidirectional associations.

  • Part 5: Handling subtype (inheritance) relationships between object types.

For reading more about the other example apps of this 5-part tutorial, see our project page.

Chapter 1. Reference Properties and Unidirectional Associations

A property defined for an object type, or class, is called a reference property if its values are references that reference an object from some class. For instance, the class Committee shown in Figure 1.1 below has a reference property chair, the values of which are references to objects of type ClubMember.

An association between object types classifies relationships between objects of those types. For instance, the association Committee-has-ClubMember-as-chair, which is visualized as a connection line in the class diagram shown in Figure 1.2 below, classifies the relationships FinanceCommittee-has-PeterMiller-as-chair, RecruitmentCommittee-has-SusanSmith-as-chair and AdvisoryCommittee-has-SarahAnderson-as-chair, where the objects PeterMiller, SusanSmith and SarahAnderson are of type ClubMember, and the objects FinanceCommittee, RecruitmentCommittee and AdvisoryCommittee are of type Committee.

Reference properties correspond to a special form of associations, namely to unidirectional binary associations. While a binary association does, in general, not need to be directional, a reference property represents a binary association that is directed from the property's domain class (where it is defined) to its range class.

In general, associations are relationship types with two or more object types participating in them. An association between two object types is called binary. In this tutorial we only discuss binary associations. For simplicity, we just say 'association' when we actually mean 'binary association'.

Table 1.1. An example of an association table

Committee-has-ClubMember-as-chair
Finance Committee Peter Miller
Recruitment Committee Susan Smith
Advisory Committee Sarah Anderson

While individual relationships (such as FinanceCommittee-has-PeterMiller-as-chair) are important information items in business communication and in information systems, associations (such as Committee-has-ClubMember-as-chair) are important elements of information models. Consequently, software applications have to implement them in a proper way, typically as part of their model layer within a model-view-controller (MVC) architecure. Unfortunately, many application development frameworks lack the required support for dealing with associations.

In mathematics, associations have been formalized in an abstract way as sets of uniform tuples, called relations. In Entity-Relationship (ER) modeling, which is the classical information modeling approach in information systems and software engineering, objects are called entities, and associations are called relationship types. The Unified Modeling Language (UML) includes the UML Class Diagram language for information modeling. In UML, object types are called classes, relationship types are called associations, and individual relationships are called "links". These three terminologies are summarized in the following table:

Table 1.2. Different terminologies

Our preferred term(s) UML ER Diagrams Mathematics
object object entity individual
object type (class) class entity type unary relation
relationship link relationship tuple
association (relationship type) association relationship type relation
functional association one-to-one, many-to-one or one-to-many relationship type function


We first discuss reference properties, which represent unidirectional binary associations in a model without any explicit graphical rendering of the association in the model diagram.

1. References and Reference Properties

A reference can be either human-readable or an internal object reference. Human-readable references refer to identifiers that are used in human communication, such as the unique names of astronomical bodies, the ISBN of books and the employee numbers of the employees of a company. Internal object references refer to the memory addresses of objects, thus providing an efficient mechanism for accessing objects in the main memory of a computer.

Some languages, like SQL and XML, support only human-readable, but not internal references. Human-readable references are called foreign keys, and the identifiers they refer to are called primary keys, in SQL. In XML, human-readable references are called ID references and the corresponding attribute type is IDREF.

Objects can be referenced either with the help of human-readable references (such as integer codes) or with internal object references, which are preferable for accessing objects efficiently in main memory. Following the XML terminology, we also call human-readable references ID references and use the suffix IdRef for the names of human-readable reference properties. When we store persistent objects in the form of records or table rows, we need to convert internal object references, stored in properties like publisher, to ID references, stored in properties like publisherIdRef. This conversion is performed as part of the serialization of the object by assigning the standard identifier value of the referenced object to the ID reference property of the referencing object.

In object-oriented languages, a property is defined for an object type, or class, which is its domain. The values of a property are either data values from some datatype, in which case the property is called an attribute, or they are object references referencing an object from some class, in which case the property is called a reference property. For instance, the class Committee shown in Figure 1.1 below has an attribute name with range String, and a reference property chair with range ClubMember.

Figure 1.1. A committee has a club member as chair expressed by the reference property chair

A committee has a club member as chair expressed by the reference property chair

Object-oriented programming languages, such as JavaScript, PHP, Java and C#, directly support the concept of reference properties, which are properties whose range is not a datatype but a reference type, or class, and whose values are object references to instances of that class.

By default, the multiplicity of a property is 1, which means that the property is mandatory and functional (or, in other words, single-valued), having exactly one value, like the property chair in class Committee shown in Figure 1.1. When a functional property is optional (not mandatory), it has the multiplicity 0..1, which means that the property's minimum cardinality is 0 and its maximum cardinality is 1.

A reference property can be either single-valued (functional) or multi-valued (non-functional). For instance, the reference property Committee::chair shown in Figure 1.1 is single-valued, since it assigns a unique club member as chair to a club. An example of a multi-valued reference property is provided by the property Book::authors shown in Figure Figure 1.10, “The association-free Publisher-Book-Author design model” below.

Normally, a multi-valued reference property is set-valued, implying that the order of the references does not matter. In certain cases, however, it may be list-valued, such that the references are ordered.

2. Referential Integrity

References are important information items in our application's database. However, they are only meaningful, when their referential integrity is maintained by the app. This requires that for any reference, there is a referenced object in the database. Consequently, any reference property p with domain class C and range class D comes with a referential integrity constraint that has to be checked whenever

  1. a new object of type C is created,

  2. the value of p is changed for some object of type C,

  3. an object of type D is destroyed.

A referential integrity constraint also implies two change dependencies:

  1. An object creation dependency: an object with a reference to another object can only be created after the referenced object has been created.

  2. An object destruction dependency: an object that is referenced by another object can only be destroyed after

    1. the referencing object is destroyed first, or

    2. the reference in the referencing object is either dropped or replaced by a another reference.

    For every reference property in our app's model classes, we have to choose, if we allow the deletion of referenced objects, and if yes, which of these two possible deletion policies applies.

In certain cases, we may want to relax this strict regime and allow creating objects that have non-referencing values for an ID reference property, but we do not consider such cases.

Typically, object creation dependencies are managed in the user interface by not allowing the user to enter a value of an ID reference property, but only to select one from a list of all existing target objects.

3. Modeling Reference Properties as Unidirectional Associations

A reference property (such as chair in the example shown in Figure 1.1 above) can be visualized in a UML class diagram in the form of an association end owned by its domain class. This requires to connect the domain class and the range class of the reference property with an association line and annotate the end of this line at the range class side with a "dot", with the property name and with a multiplicity symbol, as shown in Figure 1.2 below for the case of our example. In this way we get a unidirectional association, the source class of which is the property's domain and the target class of which is the property's range.

The fact that an association end is owned by the class at the other end is visually expressed with the help of a small filled circle (also called a "dot") at the end of the association line. This is illustrated in Figure 1.2 below, where the "dot" at the association end chair indicates that the association end represents a reference property chair in the class Committee having ClubMember as range.

Figure 1.2. A committee has a club member as chair expressed by an association end with a "dot"

A committee has a club member as chair expressed by an association end with a "dot"

Thus, the two diagrams shown in Figure 1.1 and Figure 1.2 express essentially equivalent models. When a reference property is modeled by an association end with a "dot", like chair in Figure 1.1, then the property's multiplicity is attached to the association end. Since in a design model, all association ends need to have a multiplicity, we also have to define a multiplicity for the other end at the side of the Committee class, which represents the inverse of the property. This multiplicity (of the inverse property) is not available in the original property description in the model shown in Figure 1.1, so it has to be added according to the intended semantics of the association. It can be obtained by answering the question "is it mandatory that any ClubMember is the chair of a Committee?" for finding the minimum cardinality and the question "can a ClubMember be the chair of more than one Committee?" for finding the maximum cardinality.

When the value of a property is a set of values from its range, the property is non-functional and its multiplicity is either 0..* or n..* where n > 0. Instead of 0..*, which means "neither mandatory nor functional", we can simply write the asterisk symbol *. The association shown in Figure 1.2 assigns at most one object of type ClubMember as chair to an object of type Committee. Consequently, it's an example of a functional association.

The following table provides an overview about the different cases of functionality of an association:

Table 1.3. Functionality types

Functionality type Meaning
one-to-one both functional and inverse functional
many-to-one functional
one-to-many inverse functional
many-to-many neither functional nor inverse functional


Notice that the directionality and the functionality type of an association are independent of each other. So, a unidirectional association can be either functional (one-to-one or many-to-one), or non-functional (one-to-many or many-to-many).

4. Representing Unidirectional Associations as Reference Properties

A unidirectional association between a source and a target class can be be represented as a reference property of the source class. For the case of a unidirectional one-to-one association, this is illustrated in Figure 1.3 below.

Figure 1.3. Representing the unidirectional association ClubMember has Committee as chairedCommittee as a reference property

Representing the unidirectional association ClubMember has Committee as chairedCommittee as a reference property
Representing the unidirectional association ClubMember has Committee as chairedCommittee as a reference property

5. Adding Directionality to a Non-Directed Association

When we make an information model in the form of a UML class diagram, we typically end up with a model containing one or more associations that do not have any ownership defined for their ends, as, for instance, in Figure 1.4 below. When there is no ownership dot at either end of an association, such as in this example, this means that the model does not specify how the association is to be represented (or realized) with the help of reference properties. Such an association does not have any direction. According to the UML 2.5 specification, the ends of such an association are "owned" by itself, and not by any of the classes participating in it.

Figure 1.4. A model of the Committee-has-ClubMember-as-chair association without ownership dots

A model of the Committee-has-ClubMember-as-chair association without ownership dots

A model without association end ownership dots is acceptable as a relational database design model, but it is incomplete as an information design model for classical object-oriented (OO) programming languages. For instance, the model of Figure 1.4 provides a relational database design with two entity tables, committees and clubmembers, and a separate one-to-one relationship table committee_has_clubmember_as_chair. But it does not provide a design for Java classes, since it does not specify how the association is to be implemented with the help of reference properties.

There are three options how to turn a model without association end ownership dots into a complete OO design model where all associations are either uni-directional or bi-directional: we can place an ownership dot at either end or at both ends of the association. Each of these three options defines a different way how to represent, or implement, the association with the help of reference properties. So, for the association shown in Figure 1.4 above, we have the following options:

  1. Place an ownership dot at the chair association end, leading to the model shown in Figure 1.2 above, which can be turned into the association-free model shown in Figure 1.1 above.

  2. Place an ownership dot at the chairedCommittee association end, leadig to the completed models shown in Figure 1.3 above.

  3. Make the association bi-directional by placing ownership dots at both association ends with the meaning that the association is implemented in a redundant manner by a pair of mutually inverse reference properties Committee::chair and ClubMember::chairedCommittee, as discussed in the next part of our 5-part tutorial.

    Figure 1.5. Modeling a bidirectional association between Committee and ClubMember

    Modeling a bidirectional association between Committee and ClubMember

So, whenever we have modeled an association, we have to make a choice, which of its ends represents a reference property and will therefore be marked with an ownership dot. It can be either one, or both. This decision also implies a decision about the navigability of the association. When an association end represents a reference property, this implies that it is navigable (via this property).

In the case of a functional association that is not one-to-one, the simplest design is obtained by defining the direction of the association according to its functionality, placing the association end ownership dot at the association end with the multiplicity 0..1 or 1 . For a non-directed one-to-one or many-to-many association, we can choose the direction, that is, at which association end to place the ownership dot.

6. Our Running Example

The model shown in Figure 1.6 below (about publishers and books) serves as our running example for a unidirectional functional association in this tutorial. Notice that it contains the unidirectional many-to-one association Book-has-Publisher.

Figure 1.6. The Publisher-Book information design model with a unidirectional association

The Publisher-Book information design model with a unidirectional association

We may also have to deal with a non-functional (multi-valued) reference property representing a unidirectional non-functional association. For instance, the unidirectional many-to-many association between Book and Author shown in Figure 1.7 below, models a multi-valued (non-functional) reference property authors.

Figure 1.7. The Publisher-Book-Author information design model with two unidirectional associations

The Publisher-Book-Author information design model with two unidirectional associations

7. Eliminating Unidirectional Associations

How to eliminate explicit unidirectional associations from an information design model by replacing them with reference properties

Since classical OO programming languages do not support assocations as first class citizens, but only classes and reference properties, which represent unidirectional associations (but without any explicit visual rendering), we have to eliminate all explicit associations for obtaining an OO design model.

7.1. The basic elimination procedure restricted to unidirectional associations

The starting point of our restricted association elimination procedure is an information design model with various kinds of uni-directional associations, such as the model shown in Figure 1.6 above. If the model still contains any non-directional associations, we first have to turn them into directional ones by making a decision on the ownership of their ends, as discussed in Section 5.

A uni-directional association connecting a source with a target class is replaced with a corresponding reference property in its source class having

  1. the same name as the association end, if there is any, otherwise it is set to the name of the target class (possibly pluralized, if the reference property is multi-valued);

  2. the target class as its range;

  3. the same multiplicity as the target association end,

  4. a uniqueness constraint if the uni-directional association is inverse functional.

This replacement procedure is illustrated for the case of a uni-directional one-to-one association in Figure 1.3 above.

For the case of a uni-directional one-to-many association, Figure 1.8 below provides an illustration of the association elimination procedure. Here, the non-functional association end at the target class Point is turned into a corresponding reference property with name points obtained as the pluralized form of the target class name

Figure 1.8. Turn a non-functional target association end into a corresponding reference property


7.2. Eliminating associations from the Publisher-Book-Author design model

In the case of our running example, the Publisher-Book-Author information design model, we have to replace both uni-directional associations with suitable reference properties. In the first step, we replace the many-to-one association Book-has-Publisher in the model of Figure 1.6 with a functional reference property publisher in the class Book, resulting in the following association-free model:

Figure 1.9. The association-free Publisher-Book design model

The association-free Publisher-Book design model

Notice that since the target association end of the Book-has-Publisher association has the multiplicity 0..1, we have to declare the new property publisher as optional by appending the multiplicity 0..1 to its name.

In the second step, we replace the many-to-many association Book-has-Author in the model of Figure 1.7 with a multi-valued reference property authors in the class Book, resulting in the following association-free model:

Figure 1.10. The association-free Publisher-Book-Author design model

The association-free Publisher-Book-Author design model

After the platform-independent association-free information design model has been completed, one or more platform-specific data models, for a choice of specific implementation platforms, can be derived from it. Such a platform-specific data model can still be expressed in the form of a UML class diagram, but it contains only modeling elements that can be directly encoded in the chosen platform. Thus, for any platform considered, the tutorial contains two sections: 1) how to make the platform-specific data model, and 2) how to encode this model.

8. Rendering Reference Properties in the User Interface

In an HTML-form-based user interface (UI), we have a correspondence between the different kinds of properties defined in the model classes of an app and the form controls used for the input and output of their values. We have to distinguish between various kinds of model class attributes, which are typically mapped to various types of HTML input fields, and reference properties, which are typically mapped to HTML select controls (selection lists). Representing reference properties in the UI with select controls, instead of input fields, prevents the user from entering invalid ID references, so it takes care of referential integrity.

In general, a single-valued reference property can always be represented by a single-select control in the UI, no matter how many objects populate the reference property's range, from which one specific choice is to be made. If the cardinality of the reference property's range is sufficiently small (say, not greater than 7), then we can also use a radio button group instead of a selection list.

A multi-valued reference property can be represented by a multiple-select control in the UI. However, this control is not really usable as soon as there are many (say, more than 20) different options to choose from because the way it renders the choice is visually too scattered. In the special case of having only a few (say, no more than 7) options, we can also use a checkbox group instead of a multiple-selection list. But for the general case of having in the UI an association list containing all associated objects choosen from the reference property's range class, we need to develop a special UI widget that allows to add (and remove) objects to (and from) a list of associated objects.

Such a selection list widget consists of

  1. a HTML list element containing the associated objects, where each list item contains a push button for removing the object from the association list;

  2. a single-select control that, in combination with a push button, allows to add a new associated object from the range of the multi-valued reference property.

Chapter 2. Implementing Unidirectional Functional Associations with BackboneJS/Parse

The three example apps that we have discussed in previous chapters, the minimal app,the validation app, and the enumeration app, have been limited to managing the data of one object type only. A real app, however, has to manage the data of several object types, which are typically related to each other in various ways. In particular, there may be associations and subtype (inheritance) relationships between object types. Handling associations and subtype relationships are advanced issues in software application engineering. They are often not sufficiently discussed in software development text books and not well supported by application development frameworks. In this part of the tutorial, we show how to deal with unidirectional associations, while bidirectional associations and subtype relationships are covered in parts 5 and 6.

We adopt the approach of model-based development, which provides a general methodology for engineering all kinds of artifacts, including data management apps. For being able to understand this tutorial, you need to understand the underlying concepts and theory. Either you first read the theory chapter on reference properties and associations, before you continue to read this tutorial chapter, or you start reading this tutorial chapter and consult the theory chapter only on demand, e.g., when you stumble upon a term that you don't know.

A unidirectional functional association is either one-to-one or many-to-one. In both cases such an association is represented, or implemented, with the help of a single-valued reference property.

In this chapter of our tutorial, we show

  1. how to derive a BackboneJS/Parse data model from an association-free information design model with (single-valued and multi-valued) reference properties representing unidirectional (functional and non-functional) associations,

  2. how to encode the data model in the form of BackboneJS/Parse model classes,

  3. how to write the view and controller code based on the model code.

1. Handling Single-Valued Reference Properties with BackboneJS/Parse

A single-valued reference property, such as the property publisher of the object type Book, allows storing internal references to objects of another type, such as Publisher. In a BackboneJS/Parse model class definition, such a property would have the default value null, as shown in the following example:

var Book = Parse.Object.extend("Book", {
  defaults: {
    isbn: "",
    title: "",
    year: 0,
    publisher: null
  },
  ...
})

With BackboneJS/Parse, we do not have to take care of converting objects with object references to records with corresponding ID references. Rather, we can simply save a BackboneJS/Parse object with its object references, as shown in the following program listing.

var Publisher = Parse.Object.extend("Publisher", ...);
// create a new Parse object of type Publisher
var bantam = new Publisher({ name: "Bantam Books", 
    address:"New York, USA"
});
// create a new Parse object of type Book
var book1 = new Book({ isbn: "0553345842", 
    title:"The Mind's I", year: 1982,
    publisher: bantam  // an object reference
});
bantam.save( null, {
  success: function () { book1.save();},
  error: function (err) { 
    console.log( err.message);},
});

When saved to the Parse datastore, single-valued reference properties, such as Book::publisher, are turned into "Pointer" columns in the corresponding Parse datastore table. This can be checked with the Parse data browser. The Parse concept of a Pointer corresponds to the general concept of an ID reference (or foreign key).

2. Make a BackboneJS/Parse Data Model

The starting point for making a BackboneJS/Parse data model is an association-free information design model like the following one:

How to make this design model has been discussed in the previous chapter (about unidirectional associations). We now show how to derive a JavaScript data model from this design model in four steps.

  1. Add a String-valued id attribute for standard IDs that are automatically generated by Parse. This should be the first property in the list of properties. It should be qualified as the standard identifier attribute with the stdid stereotype, and with two special constraint keywords: auto-generated for indicating that the attribute is predefined and its values are automatically generated, and frozen for indicating that the attribute's value cannot be changed.

  2. Since Parse is using the predefined id attribute as a standard identifier attribute, the standard identifier attribute of the design model class has to be either replaced by it or it has to be turned into a merely unique attribute by dropping the standard identifier (stdid) declaration of the design model class and adding a uniqueness constraint instead (so it's still a key, but no longer the primary key). The choice of these two options depends on the fact if the standard identifier attribute of the design model class represents a business concept (such as the unqie name of a publisher or the ISBN of a book), or if its values are artificial/technical identifiers without any business meaning (such as in the case of Author::personId). In the former case the attribute has to be preserved, while in the latter case it can be dropped and the id attribute can be used instead.

  3. Add the two Date-valued attributes createdAt and updatedAt, and mark the first one with the special constraint keywords auto-generated, frozen, and the second one with auto-generated, read-only for indicating that they are predefined and automatically filled by Parse.

  4. Create a check operation for each non-derived property from the design model in order to have a central place for implementing property constraints. For a reference property, such as Book::publisher, the check operation, Book.checkPublisher, has to check the corresponding referential integrity constraint, and possibly also a mandatory value constraint, if the property is mandatory.

This leads to the following BackboneJS/Parse data model class Book, where the class-level methods are shown underlined:

We have to perform a similar transformation also for the class Publisher. This gives us the complete JavaScript data model derived from the above association-free model, as depicted in the following class diagram.

Figure 2.1.  The complete BackboneJS/Parse data model


3. New issues

Compared to the single-class app discussed in Part 2 (and Part 3), we have to deal with a number of new technical issues:

  1. In the model code we now have to take care of reference properties that require

    1. validation of referential integrity constraints (ID references),

    2. to choose and implement one of the two possible deletion policies discussed in 2 for managing the corresponding object destruction dependency in the destroy method of the property's range class.

  2. In the user interface ("view") code we now have to take care of

    1. showing information about associated objects in the list objects use case;

    2. allowing to select an associated object from a list of all existing instances of the target class in the create object and update object use cases.

The last issue, allowing to select an associated object from a list of all existing instances of some class, can be solved with the help of an HTML select form element.

4. Write the Model Code

How to Encode a BackboneJS/Parse Data Model

The BackboneJS/Parse data model can be directly encoded for getting the code of the model layer of our app.

4.1. Summary

  1. Encode each model class as an extension of Parse.Object.

  2. Encode all reference properties in the form of ordinary JS properties referencing objects that instantiate a BackboneJS/Parse model class.

  3. Encode the property checks in the form of class-level ('static') methods of the model class concerned.

  4. Make sure that the property checks are invoked in the validate function of the BackboneJS/Parse model class concerned.

  5. Encode any other operation.

These steps are discussed in more detail in the following sections.

4.2. Encode each model class as an extension of Parse.Object

Each class C of the data model is encoded by means of a corresponding BackboneJS/Parse model class with the same name C having a single parameter slots, which has a key-value slot providing a value for each non-derived property of the class. The range of these properties should be indicated in a comment.

The class Book is encoded by extending Parse.Object (more specifically, as a JS object created with Parse.Object.extend). The BackboneJS/Parse model class definition from the minimal app discussed in Part 1 is extended by adding the BackboneJS methods validate and initialize:

var Book = Parse.Object.extend("Book", {
  defaults: {
    isbn: "",
    title: "",
    year: 0,
    publisher: null,
    authors: null
  },
  validate: function (slots) {
    var errMsgs = {};
    var existingRecord = Book.instances.get(this.id);
    if (!existingRecord) {  // use case Create
      errMsgs["isbn"] = Book.checkIsbn( slots.isbn).message;
    }
    if (errMsgs["isbn"]) return errMsgs;
  },
  initialize: function () {
    this.on("invalid", function( book, errMsgs){
      console.log( "Validation error(s) for "+ book.get("isbn") +": "+ 
          errMsgs["isbn"] ? "\n"+ errMsgs["isbn"] : ""
      );
    });
  },
  toString: function () {
    return "Book{ ISBN:" + this.get("isbn") + ", Title:" + 
        this.get("title") +"}"; 
  }},
  { // class-level methods
  isUniqueIsbn: function (newIsbn) {
    return Book.instances.every( function (book) {
      return newIsbn !== book.get("isbn");
    })
  }
});

As in the previous example apps, we make the simplifying assumption that we can load all data from the remote Parse data store into main memory on application start up. The purpose of the class-level property Book.instances is to hold the collection of all Book instances managed by the application in the form of a Parse collection obtained from a Parse query retrieving all Book instances:

Book.instances = (new Parse.Query( Book)).collection(); 

For each model class C, we define a class-level property C.instances representing the collection of all C instances managed by the application in the form of a JSON table (a map of records): This property is initially set to {}.

4.3. Encode the property checks

Take care that all constraints of a property as specified in the data model are properly encoded in its check function, as explained in Part 2 (Validation Tutorial). Error classes are defined in the file lib/errorTypes.js.

For instance, for the checkName operation we obtain the following code:

Publisher.checkName = function (n) {
  if (n === undefined) {
    return new MandatoryValueConstraintViolation(
        "A value for the name must be provided!");
  } else if (typeof(n) !== "string" || n.trim() === "") {
    return new RangeConstraintViolation("The name must be a non-empty string!");
  } else if (!Publisher.isUniqueName(n)) {  
    return new UniquenessConstraintViolation(
        "There is already a publisher record with this name!");
  } else {
    return new NoConstraintViolation();
  }
};

4.4. Implement a deletion policy

For any reference property, we have to choose and implement one of the two possible deletion policies discussed in 2 for managing the corresponding object destruction dependency in the destroy method of the property's range class. In our case, we have to choose between

  1. deleting all books published by the deleted publisher;

  2. dropping from all books published by the deleted publisher the reference to the deleted publisher.

We go for the second option. This is shown in the following code of the Publisher.destroy method where for all concerned book objects book the property book.publisher is cleared:

Publisher.destroy = function (id) {
  var publisher = Publisher.instances.get(id);
  publisher.destroy({
    success: function (obj) {
      console.log("Publisher " + obj.get("name") + " deleted.");
    },
    error: function (obj, error) {
      console.log("destroy error "+ error.code +": "+ error.message);
    }
  });
};

5. The View and Controller Layers

The user interface (UI) consists of a start page for navigating to the data management UI pages, one for each object type (in our example, books.html and publishers.html). Each of these data management UI pages contains 5 sections, such as manage books, list books, create book, update book and delete book, such that only one of them is displayed at any time (by setting the CSS property display:none for all others).

5.1. Initialize the app

For initializing the data management use cases, the required data (all publisher and book records) are loaded from persistent storage. This is performed in a controller procedure such as pl.ctrl.books.manage.initialize in ctrl/books.js with the following code:

pl.ctrl.books.manage = {
  initialize: function () {
    Publisher.loadAll();
    Book.loadAll();
    pl.view.books.manage.setUpUserInterface();
  }
};

The initialize method for managing book data loads the publisher table and the book table since the book data management UI needs to provide selection list for both object types. Then the menu for book data managemetn options is set up by the setUpUserInterface method.

5.2. Show information about associated objects in the List Objects use case

In our example we have only one reference property, Book::publisher, which is functional. For showing information about the optional publisher of a book in the list books use case, the corresponding cell in the HTML table is filled with the name of the publisher, if there is any:

pl.view.books.list = {
  setupUserInterface: function () {
    var tableBodyEl = document.querySelector(
                      "section#Book-R>table>tbody");
    var keys = Object.keys( Book.instances);
    var row=null, listEl=null, book=null;
    tableBodyEl.innerHTML = "";
    for (var i=0; i < keys.length; i++) {
      book = Book.instances[keys[i]];
      row = tableBodyEl.insertRow(-1);
      row.insertCell(-1).textContent = book.isbn;
      row.insertCell(-1).textContent = book.title;
      row.insertCell(-1).textContent = book.year;
      row.insertCell(-1).textContent = 
          book.publisher ? book.publisher.name : "";
    }
    document.getElementById("Book-M").style.display = "none";
    document.getElementById("Book-R").style.display = "block";
  }
};

For a multi-valued reference property, the table cell would have to be filled with a list of all associated objects referenced by the property.

5.3. Allow selecting associated objects in the create and update use cases

For allowing to select objects to be associated with the currently edited object from a list in the create and update use cases, an HTML selection list (a select element) is populated with the instances of the associated object type with the help of a utility method fillSelectWithOptions. In the case of the create book use case, the UI is set up by the following procedure:

pl.view.books.create = {
  setupUserInterface: function () {
    var formEl = document.querySelector("section#Book-C > form"),
        publisherSelectEl = formEl.selectPublisher,
        submitButton = formEl.commit;
    // define event handlers for responsive validation 
    formEl.isbn.addEventListener("input", function () {
      formEl.isbn.setCustomValidity( 
          Book.checkIsbnAsId( formEl.isbn.value).message);
    });
    // set up the publisher selection list
    util.fillSelectWithOptions( publisherSelectEl, Publisher.instances, "name");
    // define event handler for submitButton click events    
    submitButton.addEventListener("click", this.handleSubmitButtonClickEvent);
    // define event handler for neutralizing the submit event
    formEl.addEventListener( 'submit', function (e) { 
      e.preventDefault();
      formEl.reset();
    });
    // replace the manageBooks form with the Book-C form
    document.getElementById("Book-M").style.display = "none";
    document.getElementById("Book-C").style.display = "block";
    formEl.reset();
  },
  handleSubmitButtonClickEvent: function () {
    ...
  }
};

When the user clicks the submit button, all form control values, including the value of the select control, are copied to a slots list, which is used as the argument for invoking the add method after all form fields have been checked for validity, as shown in the following program listing:

  handleSubmitButtonClickEvent: function () {
    var formEl = document.querySelector("section#Book-C > form");
    var slots = {
        isbn: formEl.isbn.value, 
        title: formEl.title.value,
        year: formEl.year.value,
        publisherIdRef: formEl.selectPublisher.value
    };
    // check input fields and show constraint violation error messages 
    formEl.isbn.setCustomValidity( Book.checkIsbnAsId( slots.isbn).message);
    /* ... (do the same with title and year) */
    // save the input data only if all of the form fields are valid
    if (formEl.checkValidity()) {
      Book.add( slots);
    }
  }

The setupUserInterface code for the update book use case is similar.

Chapter 3. Implementing Unidirectional Non-Functional Associations with BackboneJS/Parse

A unidirectional non-functional association is either one-to-many or many-to-many. In both cases such an association is represented, or implemented, with the help of a multi-valued reference property.

In this chapter, we show

  1. how to derive a JavaScript data model from an association-free information design model with multi-valued reference properties representing unidirectional non-functional associations,

  2. how to encode the JavaScript data model in the form of JavaScript model classes,

  3. how to write the view and controller code based on the model code.

1. Handling Multi-Valued Reference Properties with BackboneJS/Parse

A multi-valued reference property, such as the property authors of the object type Book, allows storing a (possibly ordered) set of internal references to objects of another type, such as Authorr. For efficiently storing objects with multi-valued reference property slots, Parse is using a specific technical concept, called "Relations", which we need to choose as the column type in the Parse data browser for implementing the multi-valued reference property authors in the Parse datastore table Book. Normally, we should do this when we defne the Parse table stuctures for an app as one of the first steps of the app development project. Inspecting the initially empty Book table with the Parse data browser gives you the following picture:

Since in BackboneJS/Parse, a Parse Relation is a (special type of) JS object, a multi-valued reference property would have the default value null in a BackboneJS/Parse model class definition, as shown in the following example:

var Book = Parse.Object.extend("Book", {
  defaults: {
    isbn: "",
    title: "",
    year: 0,
    authors: null
  },
  ...
})

For assigning authors to a book, we first need to create and save them as Parse objects. So, we define an Author class, similar to Book above:

var Author = Parse.Object.extend("Author",
  ...
);

Then we create two Author objects and save them to the Parse datastore:

// create two new Parse objects of type Author
var dennett = new Author({ name: "Daniel Dennett", 
    dateOfBirth: new Date("1942-03-28")
});
var hofstadter = new Author({ name: "Douglas Hofstadter", 
    dateOfBirth: new Date("1945-02-15")
});
Parse.Object.saveAll([ dennett, hofstadter]);

We also create a new Book object, in the first step just with basic data, and not yet with authors ID references, which we can only add in a second step. after the object has been assigned a Parse object ID through saving it to the Parse datastore.

// create a new Parse object of type Book
var book1 = new Book({ isbn: "0553345842", 
    title:"The Mind's I", year: 1982
});
book1.save( null, {
  success: function () {
    // now book1 has an id value, and we can add "relations" to it
    book1.relation("authors").add( [ dennett, hofstadter]);
    book1.save();
  },
  error: function (err) { 
    console.log("save error "+ error.code + ": "+ error.message);
  }
});

2. Make a Parse/BackboneJS Data Model

The starting point for making a data model is an association-free information design model where reference properties represent unidirectional associations. The following model for our example app contains the multi-valued reference property Book::authors, which implements the unidirectional many-to-many association Book-has-Author:

How to make this design model has been discussed in the previous chapter (about unidirectional associations). We now show how to derive a Parse/BackboneJS data model from this design model. Essentially, for each class,

  1. Add a String-valued id attribute for standard IDs that are automatically generated by Parse. This should be the first property in the list of properties. It should be qualified as the standard identifier attribute with the stdid stereotype, and with the special constraint keyword auto-generated for indicating that the attribute is predefined and its values are automatically generated.

  2. Since Parse is using the predefined id attribute as a standard identifier attribute, the standard identifier attribute of the design model class has to be either replaced by it or it has to be turned into a merely unique attribute by dropping the standard identifier (stdid) declaration of the design model class and adding a uniqueness constraint instead (so it's still a key, but no longer the primary key). The choice of these two options depends on the fact if the standard identifier attribute of the design model class represents a business concept (such as the unqie name of a publisher or the ISBN of a book), or if its values are artificial/technical identifiers without any business meaning (such as in the case of Author::personId). In the former case the attribute has to be preserved, while in the latter case it can be dropped and the id attribute can be used instead.

  3. Add the two Date-valued attributes createdAt and updatedAt, and mark them both with the special constraint keywords auto-generated, read-only for indicating that they are predefined and automatically filled by Parse.

  4. Create a check operation for each non-derived property of the design model in order to have a central place for implementing all the property constraints defined in the design model.

This leads to the following data model:

3. New issues

Compared to dealing with a unidirectional functional association, as discussed in the previous chapter, we have to deal with the following new technical issues:

  1. In the model code we now have to take care of multi-valued reference properties that are implemented as a Parse Relation property.

  2. In the user interface ("view") code we now have to take care of

    1. showing information about a set of associated objects in the list objects use case;

    2. allowing to select a set of associated objects from a list of all existing instances of the target class in the create object and update object use cases.

The last issue, allowing to select a set of associated objects from a list of all existing instances of some class, can, in general, not be solved with the help of an HTML select multiple form element because of usability problems. Whenever the set of selectable options is greater than a certain threshold (defined by the number of options that can be seen on the screen without scrolling), the HTML select multiple element is no longer usable, and an alternative multi-selection widget has to be used.

4. Write the Model Code

5. Write the User Interface Code

5.1. Show information about associated objects in the List Objects use case

For showing information about the authors of a book in the list books use case, the corresponding cell in the HTML table is filled with a list of the names of all authors with the help of the utility function util.createListFromMap:

pl.view.books.list = {
  setupUserInterface: function () {
    var tableBodyEl = document.querySelector(
            "section#Book-R>table>tbody");
    var row=null, book=null, listEl=null, 
        keys = Object.keys( Book.instances);
    tableBodyEl.innerHTML = "";  // drop old contents
    for (i=0; i < keys.length; i++) {
      book = Book.instances[keys[i]];
      row = tableBodyEl.insertRow(-1);
      row.insertCell(-1).textContent = book.isbn;
      row.insertCell(-1).textContent = book.title;
      row.insertCell(-1).textContent = book.year;
      // create list of authors
      listEl = util.createListFromMap( 
          book.authors, "name");
      row.insertCell(-1).appendChild( listEl);
      row.insertCell(-1).textContent = 
          book.publisher ? book.publisher.name : "";
    }
    document.getElementById("Book-M").style.display = "none";
    document.getElementById("Book-R").style.display = "block";
  }
};

The utility function util.createListFromMap has the following code:

createListFromMap: function (aa, displayProp) {
  var listEl = document.createElement("ul");
  util.fillListFromMap( listEl, aa, displayProp);
  return listEl;
},
fillListFromMap: function (listEl, aa, displayProp) {
  var keys=[], listItemEl=null;
  // drop old contents
  listEl.innerHTML = "";
  // create list items from object property values
  keys = Object.keys( aa);
  for (var j=0; j < keys.length; j++) {
    listItemEl = document.createElement("li");
    listItemEl.textContent = aa[keys[j]][displayProp];
    listEl.appendChild( listItemEl);
  }
},

5.2. Allow selecting associated objects in the create use case

For allowing to select multiple authors to be associated with the currently edited book in the create book use case, a multiple selection list (a select element with multiple="multiple"), as shown in the HTML code below, is populated with the instances of the associated object type.

<section id="Book-C" class="UI-Page">
  <h1>Public Library: Create a new book record</h1>
  <form>
    <div class="field">
      <label>ISBN: <input type="text" name="isbn" /></label>
    </div>
    <div class="field">
      <label>Title: <input type="text" name="title" /></label>
    </div>
    <div class="field">
      <label>Year: <input type="text" name="year" /></label>
    </div>
    <div class="select-one">
      <label>Publisher: <select name="selectPublisher"></select></label>
    </div>
    <div class="select-many">
      <label>Authors: 
        <select name="selectAuthors" multiple="multiple"></select>
      </label>
    </div>
    <div class="button-group">
      <button type="submit" name="commit">Save</button>
      <button type="button" onclick="pl.view.books.manage.refreshUI()">
       Back to menu</button>
    </div>
  </form>
</section>

The create book UI is set up by populating selection lists for selecting the authors and the publisher with the help of a utility method fillSelectWithOptions as shown in the following program listing:

pl.view.books.create = {
  setupUserInterface: function () {
    var formEl = document.querySelector("section#Book-C > form"),
        publisherSelectEl = formEl.selectPublisher,
        submitButton = formEl.commit;
    // define event handlers for form field input events
      ...
    // set up the (multiple) authors selection list
    util.fillSelectWithOptions( authorsSelectEl, 
        Author.instances, "authorId", {displayProp:"name"});
    // set up the publisher selection list
    util.fillSelectWithOptions( publisherSelectEl, 
        Publisher.instances, "name");
    ...
  },
  handleSubmitButtonClickEvent: function () {
    ...
  }
};

When the user clicks the submit button, all form control values, including the value of any single-select control, are copied to a corresponding slots record variable, which is used as the argument for invoking the add method after all form fields have been checked for validity. Before invoking add, we first have to create (in the authorsIdRef slot) a list of author ID references from the selected options of the multiple author selection list, as shown in the following program listing:

  handleSubmitButtonClickEvent: function () {
    var i=0, 
        formEl = document.querySelector("section#Book-C > form"),
        selectedAuthorsOptions = formEl.selectAuthors.selectedOptions;
    var slots = {
        isbn: formEl.isbn.value, 
        title: formEl.title.value,
        year: formEl.year.value,
        authorsIdRef: [],
        publisherIdRef: formEl.selectPublisher.value
    };
    // check all input fields 
    ...
    // save the input data only if all of the form fields are valid
    if (formEl.checkValidity()) {
      // construct the list of author ID references
      for (i=0; i < selectedAuthorsOptions.length; i++) {
        slots.authorsIdRef.push( selectedAuthorsOptions[i].value);
      } 
      Book.add( slots);
    }
  }

The update book use case is discussed in the next section.

5.3. Allow selecting associated objects in the update use case

Unfortunatley, the multiple-select control is not really usable for displaying and allowig to maintain the set of associated authors in realistic use cases where we have several hundreds or thousands of authors, because the way it renders the choice is visually too scattered. So we better use a special association list widget that allows to add (and remove) objects to (and from) a list of associated objects. In order to show how this widget can replace the multiple-selection list discussed in the previous section, we use it now in the update book use case.

For allowing to maintain the set of authors associated with the currently edited book in the update book use case, an association list widget as shown in the HTML code below, is populated with the instances of the Author class.

<section id="Book-U" class="UI-Page">
  <h1>Public Library: Update a book record</h1>
  <form>
    <div class="select-one">
      <label>Select book: <select name="selectBook"></select></label>
    </div>
    <div class="field">
      <label>ISBN: <output name="isbn"></output></label>
    </div>
    <div class="field">
      <label>Title: <input type="text" name="title" /></label>
    </div>
    <div class="field">
      <label>Year: <input type="text" name="year" /></label>
    </div>
    <div class="select-one">
      <label>Publisher: <select name="selectPublisher"></select></label>
    </div>
    <div class="widget">
      <label for="updBookSelectAuthors">Authors: </label>
      <div class="MultiSelectionWidget" id="updBookSelectAuthors"></div>
    </div>
    <div class="button-group">
      <button type="submit" name="commit">Save</button>
      <button type="button" 
       onclick="pl.view.books.manage.refreshUI()">Back to menu</button>
    </div>
  </form>
</section>

The update book UI is set up (in the setupUserInterface procedure shown below) by populating

  1. the selection list for selecting the book to be updated with the help of the utility method fillSelectWithOptions, and

  2. the selection list for updating the publisher with the help of the utility method fillSelectWithOptions,

while the association list widget for updating the associated authors of the book is only populated (in handleSubmitButtonClickEvent) when a book to be updated has been chosen.

pl.view.books.update = {
  setupUserInterface: function () {
    var formEl = document.querySelector("section#Book-U > form"),
        bookSelectEl = formEl.selectBook,
        publisherSelectEl = formEl.selectPublisher,
        submitButton = formEl.commit;
    // set up the book selection list
    util.fillSelectWithOptions( bookSelectEl, Book.instances, 
        "isbn", {displayProp:"title"});
    bookSelectEl.addEventListener("change", this.handleBookSelectChangeEvent);
    ... // define event handlers for title and year input events
    // set up the associated publisher selection list
    util.fillSelectWithOptions( publisherSelectEl, Publisher.instances, "name");
    // define event handler for submitButton click events    
    submitButton.addEventListener("click", this.handleSubmitButtonClickEvent);
    // define event handler for neutralizing the submit event and reseting the form
    formEl.addEventListener( 'submit', function (e) {
      var authorsSelWidget = document.querySelector(
              "section#Book-U > form .MultiSelectionWidget");
      e.preventDefault();
      authorsSelWidget.innerHTML = "";
      formEl.reset();
    });
    document.getElementById("Book-M").style.display = "none";
    document.getElementById("Book-U").style.display = "block";
    formEl.reset();
  },

When a book to be updated has been chosen, the form input fields isbn, title and year, and the select control for updating the publisher, are assigned corresponding values from the chosen book, and the associated authors selection widget is set up:

handleBookSelectChangeEvent: function () {
  var formEl = document.querySelector("section#Book-U > form"),
      authorsSelWidget = formEl.querySelector(
          ".MultiSelectionWidget"),
      key = formEl.selectBook.value,
      book=null;
  if (key !== "") {
    book = Book.instances[key];
    formEl.isbn.value = book.isbn;
    formEl.title.value = book.title;
    formEl.year.value = book.year;
    // set up the associated authors selection widget
    util.createMultiSelectionWidget( authorsSelWidget, 
        book.authors, Author.instances, "authorId", "name");
    // assign associated publisher to index of select element  
    formEl.selectPublisher.selectedIndex = 
        (book.publisher) ? book.publisher.index : 0;
  } else {
    formEl.reset();
    formEl.selectPublisher.selectedIndex = 0;
  }
},

When the user, after updating some values, finally clicks the submit button, all form control values, including the value of the single-select control for assigning a publisher, are copied to corresponding slots in a slots record variable, which is used as the argument for invoking the update method after all values have been checked for validity. Before invoking update, a list of ID references to authors to be added, and another list of ID references to authors to be removed, is created (in the authorsIdRefToAdd and authorsIdRefToRemove slots) from the updates that have been recorded in the associated authors selection widget with the help of classList values, as shown in the following program listing:

handleSubmitButtonClickEvent: function () {
  var i=0, assocAuthorListItemEl=null, 
      authorsIdRefToAdd=[], authorsIdRefToRemove=[],
      formEl = document.querySelector("section#Book-U > form"),
      authorsSelWidget = 
          formEl.querySelector(".MultiSelectionWidget"),
      authorsAssocListEl = authorsSelWidget.firstElementChild;
  var slots = { isbn: formEl.isbn.value, 
        title: formEl.title.value,
        year: formEl.year.value,
        publisherIdRef: formEl.selectPublisher.value
      };
  // commit the update only if all of the form fields values are valid
  if (formEl.checkValidity()) {
    // construct authorsIdRefToAdd and authorsIdRefToRemove
    for (i=0; i < authorsAssocListEl.children.length; i++) {
      assocAuthorListItemEl = authorsAssocListEl.children[i]; 
      if (assocAuthorListItemEl.classList.contains("removed")) {
        authorsIdRefToRemove.push( 
            assocAuthorListItemEl.getAttribute("data-value"));          
      }
      if (assocAuthorListItemEl.classList.contains("added")) {
        authorsIdRefToAdd.push( 
            assocAuthorListItemEl.getAttribute("data-value"));          
      }
    } 
    // if the add/remove list is non-empty create a corresponding slot
    if (authorsIdRefToRemove.length > 0) {
      slots.authorsIdRefToRemove = authorsIdRefToRemove;
    }
    if (authorsIdRefToAdd.length > 0) {
      slots.authorsIdRefToAdd = authorsIdRefToAdd;
    }
    Book.update( slots);
  }
}

6. Run the App and Get the Code

You can run the example app from our server or download it as a ZIP archive file.

7. Points of Attention

Notice that in this tutorial, we have made the assumption that all application data can be loaded into main memory (like all book data is loaded into the map Book.instances). This approach only works in the case of local data storage of smaller databases, say, with not more than 2 MB of data, roughgly corresponding to 10 tables with an average population of 1000 rows, each having an average size of 200 Bytes. When larger databases are to be managed, or when data is stored remotely, it's no longer possible to load the entire population of all tables into main memory, but we have to use a technique where only parts of the table contents are loaded.

We have still included the repetitive code structures (called boilerplate code) in the model layer per class and per property for constraint validation (checks and setters) and per class for the data storage management methods add, update, and destroy. While it is good to write this code a few times for learning app development, you don't want to write it again and again later when you work on real projects. In Part 6, we will present an approach how to put these methods in a generic form in a meta-class called mODELcLASS, such that they can be reused in all model classes of an app.