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.
• Lazily retrieve (a set of) related models through the fetchRelated(key<string>, [options<object>], update<bool>) 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)),
  • reset of a HasMany relation (bind to reset:<key>; arguments: (relatedCollection)),
  • changes to the key itself on HasMany and HasOne relations (bind to update:<key>; arguments=(model, relatedModel/relatedCollection)).

• Getting started
• Backbone.Relation options
• Backbone.RelationalModel
• Example
• Known problems and solutions
• Under the hood

Resources to get you started with Backbone-relational:

• A great tutorial by antoviaque (and the accompanying git repository)

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 0.9.2 (or newer) and Underscore 1.3.1 (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. References an attribute on the data used to instantiate relatedModel.

Used to override key when determining what data to use when (de)serializing a relation, since the data backing your relations may use different naming conventions. For example, a Rails backend may provide the keys suffixed with _id or _ids. The behavior for keySource corresponds to the following rules:

1. When a relation is instantiated, the contents of the keySource are used as it's initial data.
2. The application uses the regular key attribute to interface with the relation and the models in it; the keySource is not available as an attribute for the model.

So you may be provided with data containing animal_ids, while you want to access this relation as zoo.get( 'animals' );.

NOTE: for backward compatibility reasons, setting keySource will set keyDestination as well. This means that when saving zoo, the animals attribute will be serialized back into the animal_ids key.

WARNING: when using a keySource, you should not use that attribute name for other purposes.

Value: a string. References an attribute to serialize relatedModel into.

Used to override key (and keySource) when determining what attribute to be written into when serializing a relation, since the server backing your relations may use different naming conventions. For example, a Rails backend may expect the keys to be suffixed with _attributes for nested attributes.

When calling toJSON on a model (either via Backbone.sync, or directly), the data in the key attribute is transformed and assigned to the keyDestination.

So you may want a relation to be serialized into the animals_attributes key, while you want to access this relation as zoo.get( 'animals' );.

WARNING: when using a keyDestination, you should not use that attribute name for other purposes.

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. If you define a url(models<Backbone.Model[]>) function on the specified collection, this enables fetchRelated to fetch all missing models in one request, instead of firing a separate request for each. See Backbone-tastypie for an example of a url function that can build a url for the collection (or a subset of models).

Value: a string or a boolean

Used to create a back reference from the Backbone.Collection used for a HasMany relation to the model on the other side of this relation. By default, the relation's key attribute will be used to create a reference to the RelationalModel instance from the generated collection. If you set collectionKey to a string, it will use that string as the reference to the RelationalModel, rather than the relation's key attribute. If you don't want this behavior at all, set collectionKey to false (or any falsy value) and this reference will not be created.

Value: an options hash or a function that accepts an instance of a Backbone.RelationalModel and returns an option hash

Used to provide options for the initialization of the collection in the "Many"-end of a HasMany relation. Can be an options hash or a function that should take the instance in the "One"-end of the "HasMany" relation and return an options hash

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

Determines how the contents of a relation will be serialized following a call to the toJSON method. If you specify a:

• Boolean: a value of true serializes the full set of attributes on the related model(s). Set to false to exclude the relation completely.
• String: include a single attribute from the related model(s). For example, 'name', or Backbone.Model.prototype.idAttribute to include ids.
• String[]: includes the specified attributes from the related model(s).

Only specifying true is cascading, meaning the relations of the model will get serialized as well!

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, events and properties.

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. Setting update to true guarantees that the model will be fetched from the server and any model that already exists in the store will be updated with the retrieved data.

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.

Several methods don't operate on model instances, but are defined on the type itself.

Initialize the relations and submodels for the model type. See the Q and A for a possible scenario where it's useful to call this method manually.

Create an instance of a model, taking into account what submodels have been defined.

Search for a model instance in the Backbone.Relational.store.

• If attributes is a string or a number, findOrCreate will just query the store and return a model if found.
• If attributes is an object, the model will be updated with attributes if found. Otherwise, a new model is created with attributes (unless options.create is explicitly set to false).

• 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>).

Properties can be defined along with the subclass prototype when extending Backbone.RelationalModel or a subclass thereof.

Value: an object. Default: {}.

A mapping that defines what submodels exist for the model (the superModel) on which subModelTypes is defined. The keys are used to match the subModelTypeAttribute when deserializing, and the values determine what type of submodel should be created for a key. When building model instances from data, we need to determine what kind of object we're dealing with in order to create instances of the right subModel type. This is done by finding the model for which the key is equal to the value of the submodelTypeAttribute attribute on the passed in data.

Each subModel is considered to be a proper submodel of its superclass (the model type you're extending), with a shared id pool. This means that when looking for an object of the supermodel's type, objects of a submodel's type can be returned as well, as long as the id matches. In effect, any relations pointing to the supermodel will look for instances of it's submodels as well.

Example:

Mammal = Animal.extend({
	subModelTypes: {
		'primate': 'Primate',
		'carnivore': 'Carnivore'
	}
});
var Primate = Mammal.extend();
var Carnivore = Mammal.extend();

var MammalCollection = AnimalCollection.extend({
	model: Mammal
});

// Create a collection that contains a 'Primate' and a 'Carnivore'.
var mammals = new MammalCollection([
	{ id: 3, species: 'chimp', type: 'primate' },
	{ id: 5, species: 'panther', type: 'carnivore' }
]);

Suppose that we have an Mammal model and a Primate model extending Mammal. If we have a Primate object with id 3, this object will be returned when we have a relation pointing to a Mammal with id 3, as Primate is regarded a specific kind of Mammal; it's just a Mammal with possibly some primate-specific properties or methods.

Note that this means that there cannot be any overlap in ids between instances of Mammal and Primate, as the Primate with id 3 will be the Mammal with id 3.

Value: a string. Default: "type".

The subModelTypeAttribute is a references an attribute on the data used to instantiate relatedModel. The attribute that will be checked to determine the type of model that should be built when a raw object of attributes is set as the related value, and if the relatedModel has one or more submodels.

See subModelTypes for more information.

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

// If a collection is created from a HasMany relation, it contains a reference
// back to the originator of the relation
ourHouse.get('occupants').livesIn; // === ourHouse

// 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();

Q: (Reverse) relations or submodels don't seem to be initialized properly (and I'm using CoffeeScript!)

A: You're probably using the syntax class MyModel extends Backbone.RelationalModel instead of MyModel = Backbone.RelationalModel.extend. This has advantages in CoffeeScript, but it also means that Backbone.Model.extend will not get called. Instead, CoffeeScript generates piece of code that would normally achieve roughly the same. However, extend is also the method that Backbone-relational overrides to set up relations and other things as you're defining your Backbone.RelationalModel subclass.

For exactly this scenario where you're not using .extend, Backbone.RelationalModel has the .setup method, that you can call manually after defining your subclass CoffeeScript-style. For example:

class MyModel extends Backbone.RelationalModel
	relations: [
		// etc
	]

MyModel.setup()

See issue #91 for more information.

Q: After a fetch, I don't get add:<key> events for nested relations.

A: This is due to Backbone.Collection.reset silencing add events. Pass fetch( {add: true} ) to bypass this problem. You may want to override Backbone.Collection.fetch for this, and also trigger an event when the fetch has finished while you're at it. Example:

var _fetch = Backbone.Collection.prototype.fetch;
Backbone.Collection.prototype.fetch = function( options ) {
	options || ( options = {} );
	_.defaults( options, { add: true } );

	// Remove old models
	this.reset();
	
	// Call 'fetch', and trigger an event when done.
	var dit = this,
		request = _fetch.call( this, options );
	request.done( function() {
			if ( !options.silent ) {
				dit.trigger( 'fetch', dit, options );
			}
		});

	return request;
};

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.