func (s *Session) FindRef(ref *DBRef) *Query {
	if ref.Database == "" {
		panic(errors.New(fmt.Sprintf("Can't resolve database for %#v", ref)))
	}
	c := s.DB(ref.Database).C(ref.Collection)
	return c.FindId(ref.Id)
}
func (s *Session) FindRef(ref *DBRef) *Query {
	if ref.Database == "" {
		panic(errors.New(fmt.Sprintf("Can't resolve database for %#v", ref)))
	}
	c := s.DB(ref.Database).C(ref.Collection)
	return c.FindId(ref.Id)
}
// FindRef returns a query that looks for the document in the provided
// reference. For a DBRef to be resolved correctly at the session level
// it must necessarily have the optional DB field defined.
//
// See also the DBRef type and the FindRef method on Database.
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Database+References
//