assets/js/vendor/backbone-relational.js
function( keyContents ) {
			this.keyContents = keyContents instanceof Backbone.Collection ? keyContents : null;
			this.keyIds = [];

			if ( !this.keyContents && ( keyContents || keyContents === 0 ) ) { // since 0 can be a valid `id` as well
				// Handle cases the an API/user supplies just an Object/id instead of an Array
				this.keyContents = _.isArray( keyContents ) ? keyContents : [ keyContents ];

				_.each( this.keyContents, function( item ) {
					var itemId = Backbone.Relational.store.resolveIdForItem( this.relatedModel, item );
					if ( itemId || itemId === 0 ) {
						this.keyIds.push( itemId );
					}
				}, this );
			}
		}
function( keyContents ) {
			this.keyContents = keyContents instanceof Backbone.Collection ? keyContents : null;
			this.keyIds = [];

			if ( !this.keyContents && ( keyContents || keyContents === 0 ) ) { // since 0 can be a valid `id` as well
				// Handle cases the an API/user supplies just an Object/id instead of an Array
				this.keyContents = _.isArray( keyContents ) ? keyContents : [ keyContents ];

				_.each( this.keyContents, function( item ) {
					var itemId = Backbone.Relational.store.resolveIdForItem( this.relatedModel, item );
					if ( itemId || itemId === 0 ) {
						this.keyIds.push( itemId );
					}
				}, this );
			}
		}
Normalize and reduce `keyContents` to a list of `ids`, for easier comparison
@param {String|Number|String[]|Number[]|Backbone.Collection} keyContents