code: func (q *Query) JoinOn(condition string, params ...interface{}) *Query {
  	if q.joinAppendOn == nil {
  		q.err(errors.New("pg: no joins to apply JoinOn"))
  		return q
  	}
  	q.joinAppendOn(&condAppender{
  		sep:    " AND ",
  		cond:   condition,
  		params: params,
  	})
  	return q
  }
docs: // JoinOn appends join condition to the last join.