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