Laravel如何获取三层深度关联的Related对象列表

针对你的模型层级（Question → hasMany Decision → hasMany Impact → morphTo Related），hasManyThrough确实只支持两层关联的穿透，这里提供几种可行的实现方案：

方案一：嵌套预加载+手动合并数据 #

这是最直观的方案，适合数据量不大的场景：

1. 先预加载所有层级的关联数据：

$questions = Question::with('decisions.impact.related')->get();

2. 遍历每个Question，逐层收集所有Related对象：

$result = $questions->map(function ($question) {
    $relateds = collect();
    // 遍历决策
    foreach ($question->decisions as $decision) {
        // 遍历每个决策对应的影响
        foreach ($decision->impact as $impact) {
            // 确保关联对象存在，避免空值
            if ($related = $impact->related) {
                $relateds->push($related);
            }
        }
    }
    return [
        'question' => $question->id,
        'relateds' => $relateds->toArray()
    ];
});

执行后就能得到你需要的question与relateds对应格式的数组。

方案二：直接通过SQL JOIN查询 #

如果数据量较大，手动合并效率偏低，可以通过数据库层面的JOIN直接聚合结果（注意多态关联需要处理related_type和related_id）：

$questions = Question::select('questions.id as question')
    // 根据你的Related模型实际字段调整JSON_OBJECT内的内容
    ->selectRaw("JSON_ARRAYAGG(JSON_OBJECT('id', r.id, 'type', i.related_type)) as relateds")
    ->join('decisions', 'decisions.question_id', '=', 'questions.id')
    ->join('impacts', 'impacts.decision_id', '=', 'decisions.id')
    // 多态关联的JOIN需要匹配id和类型
    ->join('related_models as r', function ($join) {
        $join->on('r.id', '=', 'impacts.related_id')
             ->whereColumn('r.type', 'impacts.related_type');
    })
    ->groupBy('questions.id')
    ->get();

注：related_models是泛指多态关联对应的表，如果你的多态关联对应多个不同表，可能需要用UNION合并不同表的数据后再关联。

方案三：自定义三层穿透关联（适合Laravel） #

如果需要在多个地方复用这个关联逻辑，可以自定义一个扩展HasManyThrough的关联类，实现三层关联的穿透：

1. 创建自定义关联类：

use Illuminate\Database\Eloquent\Relations\HasManyThrough;
use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Model;

class HasManyThroughMany extends HasManyThrough
{
    protected $secondThroughModel;
    protected $thirdKey;

    public function __construct(
        Builder $query,
        Model $parent,
        Model $through,
        Model $secondThrough,
        string $firstKey,
        string $secondKey,
        string $thirdKey,
        string $localKey
    ) {
        parent::__construct($query, $parent, $through, $firstKey, $secondKey, $localKey);
        $this->secondThroughModel = $secondThrough;
        $this->thirdKey = $thirdKey;
    }

    // 重写关联查询逻辑，添加第二层中间表的关联
    protected function getRelationExistenceQuery(Builder $query, Builder $parentQuery, $columns = ['*'])
    {
        $query = parent::getRelationExistenceQuery($query, $parentQuery, $columns);
        $secondThroughTable = $this->secondThroughModel->getTable();
        return $query->join(
            $secondThroughTable,
            "{$this->through->getTable()}.{$this->secondKey}",
            '=',
            "{$secondThroughTable}.{$this->localKey}"
        )->whereColumn(
            "{$secondThroughTable}.{$this->thirdKey}",
            '=',
            "{$this->related->getTable()}.{$this->related->getKeyName()}"
        );
    }
}

2. 在Question模型中定义这个关联：

public function relateds()
{
    return $this->hasManyThroughMany(
        Related::class,
        Decision::class,
        Impact::class,
        'question_id', // Question -> Decision 的外键
        'decision_id', // Decision -> Impact 的外键
        'related_id', // Impact -> Related 的外键
        'id' // Question的主键
    );
}

之后就可以像普通关联一样使用：$questions = Question::with('relateds')->get();，再自行整理成需要的格式即可。

内容的提问来源于stack exchange，提问作者anderlaini