How to return one instance of a eloquent many to many relation with eager loading?

问题

class Parent extends Model
{
    public function kids()
    {
        return $this->belongsToMany('App\Models\Kid')
            ->orderBy('age')
            ->withTimestamps();
    }

    public function oldestKid()
    {
        return $this->belongsToMany('App\Models\Kid')
            ->orderByDesc('age')
            ->take(1);
    }
}

Problem with this approach is that $parent->oldestKid returns an array. It would feel more logical if it would return an object.

$parent = App\Models\Parent::with('oldestKid')->first();

回答1:

This is what we ended up with, and its working.

Important to add: the pivot table is kid_parent


public function oldestKid()
{
    return $this->belongsTo(Kid::class, 'oldest_kid_id', 'id');
}

public function scopeWithOldestKid($query)
{
    $query->addSelect(['oldest_kid_id' => KidParent::select('kid_id')
        ->whereColumn('parent_id', 'parents.id')
        ->join('kids', 'kids.id', '=', 'kid_parent.kid_id')
        ->orderByDesc('kids.age')
        ->take(1)
    ])->with('oldestKid');
}

then you can use it like that :

$parents = Parent::withOldestKid()->get();

foreach($parents as $parent){
 $oldest_kid = $parent->oldestKid;
 
}

If you want to go crazy: you can use https://laravel.com/docs/8.x/eloquent#global-scopes so it is always loaded in if you go for a parent.

回答2:

You have to use a subquery to do that :

    public function oldestKid()
    {
        return $this->belongsTo(Kid::class);
    }

    public function scopeWithOldestKid($query)
    {
        $query->addSelect(['oldest_kid_id' => Kid::select('id')
            ->whereColumn('parent_id', 'parents.id')
            -> orderByDesc('age')
            ->take(1)
        ])->with('oldestKid');
    }

then you can use it like that :


$parents = Parent::withOldestKid()->get();

foreach($parents as $parent){
 $oldest_kid = $parent->oldestKid;
 
}

来源：https://stackoverflow.com/questions/64440891/how-to-return-one-instance-of-a-eloquent-many-to-many-relation-with-eager-loadin

标签

php

laravel

eloquent