Collection instances share `options` object by reference which is unexpected

[lukasmlady]

Minimal case showing the problem

Following is a test case that proves the equality in a minimalistic case:

const Child = Backbone.RelationalModel.extend();
const Children = Backbone.Collection.extend({ model: Child });
const Parent = Backbone.RelationalModel.extend({
  relations: [{
    type: Backbone.HasMany,
    key: 'children',
    relatedModel: Child,
    relatedCollection: Children
  }]
});

const parent1 = new Parent({ id: 'parent1', children: [{ id: 'child1' }] });
const parent2 = new Parent({ id: 'parent2', children: [{ id: 'child2' }] });

// This is how it is, though not really expected.
expect(parent1.get('children').options).to.equal(parent2.get('children').options);

Potential cause

In the following snippet, collectionOptions gets assigned an empty object, once when loading the library.

Backbone-relational/backbone-relational.js

Lines 932 to 942 in 1ee7dc0

	module.HasMany = module.Relation.extend({
	collectionType: null,
	options: {
	reverseRelation: { type: 'HasOne' },
	collectionType: module.Collection,
	collectionKey: true,
	collectionOptions: {}
	},
	initialize: function( opts ) {

That object is then reused and passed (in the case it is not a function) as options when instantiating a new collection.

Backbone-relational/backbone-relational.js

Lines 967 to 977 in 1ee7dc0

	_prepareCollection: function( collection ) {
	if ( this.related ) {
	this.stopListening( this.related );
	}
	if ( !collection || !( collection instanceof module.Collection ) ) {
	var options = _.isFunction( this.options.collectionOptions ) ?
	this.options.collectionOptions( this.instance ) : this.options.collectionOptions;
	collection = new this.collectionType( null, options );
	}

This most likely leads to instances of collections sharing a reference to the very same this.options object.