thinkphp5提供了withCount()方法来封装withSum()，并调用

来封装hasMany()、hasOne()等。如果有两张表，user（用户表）和order（订单表），我们需要对每个用户订单中的fee字段进行求和。

Query.php 添加

//统计求和
    public function withSum($relation, $subQuery = true, $sum='sum')
    {
        if (!$subQuery) {
            $this->options['with_sum'] = $relation;
        } else {
            $relations = is_string($relation) ? explode(',', $relation) : $relation;
            if (!isset($this->options['field'])) {
                $this->field('*');
            }
            foreach ($relations as $key => $relation) {
                $closure = false;
                if ($relation instanceof \Closure) {
                    $closure  = $relation;
                    $relation = $key;
                }
                $relation = Loader::parseName($relation, 1, false);
                $count    = '(' . $this->model->$relation()->getRelationSumQuery($closure, $sum) . ')';
                $this->field([$count => Loader::parseName($relation) . '_sum']);
            }
        }
        return $this;
    }

HasMany.php 添加：

public function getRelationSumQuery($closure, $sum)
    {
        if ($closure) {
            call_user_func_array($closure, [ & $this->query]);
        }
        $localKey = $this->localKey ?: $this->parent->getPk();
        return $this->query->whereExp($this->foreignKey, '=' . $this->parent->getTable() . '.' . $localKey)->fetchSql()->sum($sum);
    }

调用方法：

 $data = model('user')->withSum('order', 'fee')->select();

widthSum('order')，其中order是指模型中为hasMany添加的方法。

    public function order()
    {
        return $this->hasMany('Order');
    }