Mongo-style reduce for Backbone collections.

backbone_extensions.js

// Reduce collection using provided conditions
// e.g. FriendList.where({age: 30, gender: 'male'})
// Returns object of the same "class", so it's possible to chain methods
Backbone.Collection.prototype.where = function(conditions) {
    return new this.constructor(_(conditions).reduce(function(memo, value, key) {
        memo = _(memo).filter(function(model) {
            return model.get(key) === value;
        });
        return memo;
    }, this.models));
};