根据id和子表parent_id，组装sql，将查询到的数据再根据关联关系附加到数据中去

例如你的数据 select * from table where parent_id in (1)得到的数据在放到list上面去

通过关联方法的 match 方法来进行匹配的，比如前面的 HasMany。

是从查询构建器的 get -> eagerLoadRelations -> eagerLoadRelation

临下班了， 特地帮你把源码翻出来，就是foreach

namespace Illuminate\Database\Eloquent\Relations;

abstract class HasOneOrMany extends Relation
{
    /**
     * Match the eagerly loaded results to their many parents.
     *
     * @param  array  $models
     * @param  \Illuminate\Database\Eloquent\Collection  $results
     * @param  string  $relation
     * @param  string  $type
     * @return array
     */
    protected function matchOneOrMany(array $models, Collection $results, $relation, $type)
    {
        $dictionary = $this->buildDictionary($results);

        // Once we have the dictionary we can simply spin through the parent models to
        // link them up with their children using the keyed dictionary to make the
        // matching very convenient and easy work. Then we'll just return them.
        foreach ($models as $model) {
            if (isset($dictionary[$key = $this->getDictionaryKey($model->getAttribute($this->localKey))])) {
                $model->setRelation(
                    $relation, $this->getRelationValue($dictionary, $key, $type)
                );
            }
        }

        return $models;
    }
}