Backbone-relational provides one-to-one, one-to-many and many-to-one relations between models for Backbone. To use relations, extend Backbone.RelationalModel (instead of the regular Backbone.Model) and define a property relations, containing an array of option objects. Each relation must define (as a minimum) the type, key and relatedModel. Available relation types are Backbone.HasOne and Backbone.HasMany. Backbone-relational features:

• Bidirectional relations, which notify related models of changes through events.
• Control how relations are serialized using the includeInJSON option.
• Automatically convert nested objects in a model's attributes into Model instances using the createModels option.
• Retrieve (a set of) related models through the fetchRelated(key<string>, [options<object>]) method.
• Determine the type of HasMany collections with collectionType.
• Bind new events to a Backbone.RelationalModel for:
  • addition to a HasMany relation (bind to add:<key>; arguments: (addedModel, relatedCollection)),
  • removal from a HasMany relation (bind to remove:<key>; arguments: (removedModel, relatedCollection)),
  • changes to the key itself on HasMany and HasOne relations (bind to update:<key>; arguments=(model, relatedModel/relatedCollection)).

• Installation
• Backbone.Relation options
• Backbone.RelationalModel
• Example
• Under the hood

Backbone-relational depends on backbone (and thus on underscore). Include Backbone-relational right after Backbone and Underscore:

<script type="text/javascript" src="./js/underscore.js"></script>
<script type="text/javascript" src="./js/backbone.js"></script>
<script type="text/javascript" src="./js/backbone-relational.js"></script>

Backbone-relational has been tested with Backbone 0.5.3 (or newer) and Underscore 1.2.1 (or newer).

Each Backbone.RelationalModel can contain an array of relations. Each relation supports a number of options, of which relatedModel, key and type are mandatory. A relation could look like the following:

Zoo = Backbone.RelationalModel.extend({
	relations: [{
			type: Backbone.HasMany,
			key: 'animals',
			relatedModel: 'Animal',
			collectionType: 'AnimalCollection',
			reverseRelation: {
				key: 'livesIn',
				includeInJSON: 'id'
				// 'relatedModel' is automatically set to 'Zoo'; the 'relationType' to 'HasOne'.
			}
		}]
});

Animal = Backbone.RelationalModel.extend({
	urlRoot: '/animal/'
});

AnimalCollection = Backbone.Collection.extend({
	model: Animal,
	
	url: function( models ) {
		return '/animal/' + ( models ? 'set/' + _.pluck( models, 'id' ).join(';') + '/' : '' );
	}
});

Value: a string (which can be resolved to an object type on the global scope), or a reference to a Backbone.RelationalModel type.

Value: a string. References an attribute name on relatedModel.

Value: a string, or a reference to a Backbone.Relation type

Example: Backbone.HasOne or 'HasMany'.

The key for a HasOne relation consists of a single Backbone.RelationalModel. The default reverseRelation.type for a HasOne relation is HasMany. This can be set to HasOne instead, to create a one-to-one relation.

The key for a HasMany relation consists of a Backbone.Collection, containing zero or more Backbone.RelationalModels. The default reverseRelation.type for a HasMany relation is HasOne; this is the only option here, since many-to-many is not supported directly.

A many-to-many relation can be modeled using two Backbone.HasMany relations, with a link model in between:

Person = Backbone.RelationalModel.extend({
	relations: [
		{
			type: 'HasMany',
			key: 'jobs',
			relatedModel: 'Job',
			reverseRelation: {
				key: 'person'
			}
		}
	]
});

// A link object between 'Person' and 'Company', to achieve many-to-many relations.
Job = Backbone.RelationalModel.extend({
	defaults: {
		'startDate': null,
		'endDate': null
	}
})

Company = Backbone.RelationalModel.extend({
	relations: [
		{
			type: 'HasMany',
			key: 'employees',
			relatedModel: 'Job',
			reverseRelation: {
				key: 'company'
			}
		}
	]
});

niceCompany = new Company( { name: 'niceCompany' } );
niceCompany.bind( 'add:employees', function( model, coll ) {
		// Will see a Job with attributes { person: paul, company: niceCompany } being added here
	});

paul.get('jobs').add( { company: niceCompany } );

Value: a string (which can be resolved to an object type on the global scope), or a reference to a Backbone.Collection type.

Determine the type of collections used for a HasMany relation. Defining a url(models<Backbone.Model[]>) function on this Collection that's able to build a url for either the whole collection, or a set of models enables fetchRelated to fetch all missing models in one request, instead of firing a separate request for each. See Backbone-tastypie for an example.

Value: a boolean, or a string referencing one of the model's attributes. Default: true.

Determines how a relation will be serialized following a call to the toJSON method. A value of true serializes the full set of attributes on the related model(s), in which case the relations of this object are serialized as well. Set to false to exclude the relation completely. You can also choose to include a single attribute from the related model by using a string. For example, 'name', or Backbone.Model.prototype.idAttribute to include ids.

Value: a boolean. Default: true.

Should models be created from nested objects, or not?

If the relation should be bidirectional, specify the details for the reverse relation here. It's only mandatory to supply a key; relatedModel is automatically set. The default type for a reverseRelation is HasMany for a HasOne relation (which can be overridden to HasOne in order to create a one-to-one relation), and HasOne for a HasMany relation. In this case, you cannot create a reverseRelation with type HasMany as well; please see Many-to-many relations on how to model these type of relations.

Please note: if you define a relation (plus a reverseRelation) on a model, but never actually create an instance of that model, the model's constructor will never run, which means it's initializeRelations will never get called, and the reverseRelation will not be initialized either. In that case, you could either define the relation on the opposite model, or define two single relations. See issue 20 for a discussion.

Backbone.RelationalModel introduces a couple of new methods and events.

Returns the set of initialized relations on the model.

Fetch models from the server that were referenced in the model's attributes, but have not been found/created yet. This can be used specifically for lazy-loading scenarios.

By default, a separate request will be fired for each additional model that is to be fetched from the server. However, if your server/API supports it, you can fetch the set of models in one request by specifying a collectionType for the relation you call fetchRelated on. The collectionType should have an overridden url(models<Backbone.Model[]>) method that allows it to construct a url for an array of models. See the example at the top of Backbone.Relation options or Backbone-tastypie for an example.

• add: triggered on addition to a HasMany relation.
  Bind to add:<key>; arguments: (addedModel<Backbone.Model>, related<Backbone.Collection>).
• remove: triggered on removal from a HasMany relation.
  Bind to remove:<key>; arguments: (removedModel<Backbone.Model>, related<Backbone.Collection>).
• update: triggered on changes to the key itself on HasMany and HasOne relations.
  Bind to update:<key>; arguments: (model<Backbone.Model>, related<Backbone.Model|Backbone.Collection>).

paul = new Person({
	id: 'person-1',
	name: 'Paul',
	user: { id: 'user-1', login: 'dude', email: '[email protected]' }
});

// A User object is automatically created from the JSON; so 'login' returns 'dude'.
paul.get('user').get('login');

ourHouse = new House({
	id: 'house-1',
	location: 'in the middle of the street',
	occupants: ['person-1', 'person-2', 'person-5']
});

// 'ourHouse.occupants' is turned into a Backbone.Collection of Persons.
// The first person in 'ourHouse.occupants' will point to 'paul'.
ourHouse.get('occupants').at(0); // === paul

// the relation from 'House.occupants' to 'Person' has been defined as a bi-directional HasMany relation,
// with a reverse relation to 'Person.livesIn'. So, 'paul.livesIn' will automatically point back to 'ourHouse'.
paul.get('livesIn'); // === ourHouse

// You can control which relations get serialized to JSON (when saving), using the 'includeInJSON'
// property on a Relation. Also, each object will only get serialized once to prevent loops.
paul.get('user').toJSON();
	/* result:
		{
			email: "[email protected]",
			id: "user-1",
			login: "dude",
			person: {
				id: "person-1",
				name: "Paul",
				livesIn: {
					id: "house-1",	
					location: "in the middle of the street",
					occupants: ["person-1"] // just the id, since 'includeInJSON' references the 'idAttribute'
				},
				user: "user-1" // not serialized because it is already in the JSON, so we won't create a loop
			}
		}
	*/

// Load occupants 'person-2' and 'person-5', which don't exist yet, from the server
ourHouse.fetchRelated( 'occupants' );

// Use the 'add' and 'remove' events to listen for additions/removals on HasMany relations (like 'House.occupants').
ourHouse.bind( 'add:occupants', function( model, coll ) {
		// create a View?
		console.debug( 'add %o', model );
	});
ourHouse.bind( 'remove:occupants', function( model, coll ) {
		// destroy a View?
		console.debug( 'remove %o', model );
	});

// Use the 'update' event to listen for changes on a HasOne relation (like 'Person.livesIn').
paul.bind( 'update:livesIn', function( model, attr ) {
		console.debug( 'update to %o', attr );
	});


// Modifying either side of a bi-directional relation updates the other side automatically.
// Make paul homeless; triggers 'remove:occupants' on ourHouse, and 'update:livesIn' on paul
ourHouse.get('occupants').remove( paul.id ); 

paul.get('livesIn'); // yup; nothing.

// Move back in; triggers 'add:occupants' on ourHouse, and 'update:livesIn' on paul
paul.set( { 'livesIn': 'house-1' } );

This is achieved using the following relations and models:

House = Backbone.RelationalModel.extend({
	// The 'relations' property, on the House's prototype. Initialized separately for each instance of House.
	// Each relation must define (as a minimum) the 'type', 'key' and 'relatedModel'. Options are
	// 'includeInJSON', 'createModels' and 'reverseRelation', which takes the same options as the relation itself.
	relations: [
		{
			type: Backbone.HasMany, // Use the type, or the string 'HasOne' or 'HasMany'.
			key: 'occupants',
			relatedModel: 'Person',
			includeInJSON: Backbone.Model.prototype.idAttribute,
			collectionType: 'PersonCollection',
			reverseRelation: {
				key: 'livesIn'
			}
		}
	]
});

Person = Backbone.RelationalModel.extend({
	relations: [
		{ // Create a (recursive) one-to-one relationship
			type: Backbone.HasOne,
			key: 'user',
			relatedModel: 'User',
			reverseRelation: {
				type: Backbone.HasOne,
				key: 'person'
			}
		}
	],
	
	initialize: function() {
		// do whatever you want :)
	}
});

PersonCollection = Backbone.Collection.extend({
	url: function( models ) {
		// Logic to create a url for the whole collection, or a set of models.
		// See the tests, or Backbone-tastypie, for an example.
		return '/person/' + ( models ? 'set/' + _.pluck( models, 'id' ).join(';') + '/' : '' );
	}
});

User = Backbone.RelationalModel.extend();

Each Backbone.RelationalModel registers itself with Backbone.Store upon creation (and is removed from the Store when destroyed). When creating or updating an attribute that is a key in a relation, removed related objects are notified of their removal, and new related objects are looked up in the Store.