Get filters of query.

filter.js

/**
 * Get filters of query.
 *
 * @param {HttpRequest} req
 * @param {Object} [options]
 *		- {Boolean} [customizable]
 *			whether the filter is customizable
 *			i.e. can be pass from query string with the key $filter
 *			e.g. $filter=id,key,value
 *		- {Object} [allow]
 *			defined fields that can be selected
 *			e.g. { id:1, key:1, value:1 }
 *		- {Object} [defaults]
 *			defined default filter of query
 *			e.g. { key:1, value:1 }
 * @returns {Object} selection part for query
 */

function __filter(req, options) {

	var result = null,
		opt = options || {};

	// customizable
	if (opt.customizable) {
		result = utils.param(req, '$filter', function(value) {
			return value ? utils.objectify(value.split(/[, ]+/)) : null;
		});

		// allow
		if (result && opt.allow) {
			for (var i in result) {
				if (!opt.allow[i]) {
					delete result[i];
				};
			};
		};
	};

	// default
	if (!result && opt.defaults) {
		result = opt.defaults;
	};

	return result;
}