code: func (m *Manager) UpdateMembership(globalid string, username string, oldrole string, newrole string) error {
  	qry := bson.M{"globalid": globalid}
  	pull := bson.M{
  		"$pull": bson.M{oldrole: username},
  	}
  	push := bson.M{
  		"$addToSet": bson.M{newrole: username},
  	}
  	err := m.collection.Update(qry, pull)
  	if err != nil {
  		return err
  	}
  	return m.collection.Update(qry, push)
  }
docs: // UpdateMembership Updates a user his role in an organization