Brainy collection using a class method.

class-method.js

define([
  'backbone'
], function(Backbone) {

  return Backbone.Collection.extend({
    url: '/boops'

  }, {

    byUserId: function(user_id) {
      var Collection = this,
        collection = new Collection;
      collection.fetch = function(options) {
        options = options || {};
        options.data = options.data || {};
        options.data.user_id = user_id;
        Collection.prototype.fetch.apply(collection, [options]);
      };
      return collection;
    }

  });

});