[8.x] MorphTo relationship eager loading constraints

[tobyzerner]

Currently it is not possible to apply different constraints to the queries that eager load each different type in a morph-to relation. You can apply constraints directly to the MorphTo relation object:

use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Relations\MorphTo;

$activities = ActivityFeed::query()
    ->with(['parentable' => function (MorphTo $query) {
        $query->where('foo', 'bar');
    }])->get();

However these constraints will be applied uniformly to the queries for each morph type, so it is only useful if constraining by a column which is present on all types – otherwise, it will fail.

In a similar vein to the morphWith method introduced in #28647, this PR introduces the ability to apply different constraints to the queries for each type:

use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Relations\MorphTo;

$activities = ActivityFeed::query()
    ->with(['parentable' => function (MorphTo $morphTo) {
        $morphTo->constrain([
            Event::class => function (Builder $query) {
                $query->where('event_foo', 'bar');
            },
            Photo::class => function (Builder $query) {
                $query->where('photo_foo', 'bar');
            },
        ]);
    }])->get();

[taylorotwell]

Thanks. Could you send a PR to the laravel/docs repository for this?

[tobyzerner]

Sure can!