think\Model::relation PHP Method

    protected function relation($relation = null)
    {
        if (!is_null($relation)) {
            // 执行关联查询
            return $this->db()->relation($relation);
        }
        // 获取关联对象实例
        if (is_null($this->relation)) {
            $this->relation = new Relation($this);
        }
        return $this->relation;
    }