Add a Django-like filter to Backbone Models and Collections

filters.js

/**
	 * Backbone filter. This adds a filtering mechanism
	 * similar to Django ORM filters:
	 * 
	 * var MyModel = Backbone.Model.extend(...);
	 * var model = new MyModel(...);
	 * model.filter("somekey", "somevalue");
	 * model.filter("anotherkey", "anothervalue");
	 * Doing so will result in the query() method to
	 * return a query string of all filters:
	 * 
	 * somekey=somevalue&anotherkey=anothervalue
	 * 
	 * Note that if the key is an instande of Backbone.Model
	 * you can take another model as the key and the query()
	 * method will translate this to get the id attribute
	 * as the value and the Model's name as the key, e.g.
	 * 
	 * var MyModel = Backbone.Model.extend(...);
	 * var User    = Backbone.Model.extend({ ..., asFilter : 'user', ...});
	 * var model = new MyModel(...);
	 * var user = new User({id : 1})).fetch();
	 * model.filter(user);
	 * model.query();
	 * => user=1
	 * 
	 * Notes: 
	 * (1) for this to work, the Model must specify the asFilter 
	 * attribute. asFilter can be a value or a function.
	 * (2) the get('id') call to get the key value of the filter
	 * is called by filter() not by query(). If you want to 
	 * override this, pass a function, which will be evaluated
	 * by query(), that is possibly much later then the filter
	 * is defined. Use this if you want the filter to adapt 
	 * dynamically.  
	 * 
	 * Use the query() function to construct the URL:
	 * 
	 * ...
	 *    url : function() {
	 * 	    return this.urlRoot + "?" + this.query();
	 * }
	 * ...
	 */
	var Filter = {
		/**
		 * add a filter
		 * 
		 * @param key   string or instance of a Backbone.Model
		 * @param value string or function. functions are evaluated
		 * at query() time
		 * @param op    undefined or Django tastypie operator
		 * @return this for chainable calls 
		 */
		filter : function(key, value, op) {
			if(key instanceof $B.Model) {
				value = _.isFunction(value) ? value : key.get('id');
				key = _.isFunction(key.asFilter) ? key.asFilter() : key.asFilter;
			} 
			this._filters[key + (op ? '__' + op : '')] = value;	
			return this;
		},
		/**
		 * clear all filters. Note this does not affect the 
		 * current state of the model, it only takes effect upon
		 * the next sync() call
		 */
		clearFilter : function() {
			this._filters = {}; 
		}
		/**
		 * returns true if one or more filters have been applied
		 * false otherwise 
		 */
		filtered : function() {
			return this.keys(_filters).length > 0;
		},
		/**
		 * return the current set of filters as a dictionary
		 * of key/value pairs
		 */
		getFilter : function() {
			return this._filters();
		},
		/**
		 * dict of filters, value can be a function
		 * or a string:
		 * {
		 * 	key : value,
		 *  key2 : function() { ... }
		 * }
		 */
		_filters : {},
		/**
		 * return the URL's query string. This method
		 * used by the default url() (see tastypie.js)
		 * @return filters as term=value&term2=value2
		 */
		query : function() {
			// create the list of filters as key value pair
			var queryfilters = [];
			for (var key in this._filters) {
				var value = _.isFunction(this._filters[key]) ? this._filters[key](this) : this._filters[key];
				queryfilters.push("{0}={1}".format([key, value]));
			};
			// return the query string
			return queryfilters.join('&');
		}
	};
	_.extend($B.Model.prototype, Filter);
	_.extend($B.Collection.prototype, Filter);