code: func (query Query) Lock(lock ...string) Query {
  	if !query.repo.inTransaction {
  		return query
  	}
  
  	if len(lock) > 0 {
  		query.LockClause = lock[0]
  	} else {
  		query.LockClause = "FOR UPDATE"
  	}
  
  	return query
  }
docs: // Lock query using pessimistic locking.
  // Lock expression can be specified as first parameter, default to FOR UPDATE.