code: 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)
  }
docs: // 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
  //