Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (c *Collection) UpdateId(id interface{}, update interface{}) error {
	return c.Update(bson.D{{"_id", id}}, update)
}
func (c *Collection) UpdateId(id interface{}, update interface{}) error {
	return c.Update(bson.D{{"_id", id}}, update)
}
// UpdateId is a convenience helper equivalent to:
//
//     err := collection.Update(bson.M{"_id": id}, update)
//
// See the Update method for more details.