[5.8] Fix MorphTo::associate() with custom owner key

[staudenmeir]

MorphTo::associate() doesn't consider a custom owner key and always uses the primary key:

class Comment extends Model
{
    public function commentable()
    {
        return $this->morphTo(null, null, null, 'slug');
    }
}

$post = Post::create(['slug' => 'foo']);

$comment = (new Comment)->commentable()->associate($post);

dump($comment->commentable_id);

// expected: "foo"
// actual:   1

[lindyhopchris]

@staudenmeir did you test this within a real app? I've got failing tests now.

The scenario is Comment can morph to Post or Video. Post and Video have different key names.

If you have a Comment that is currently associated to a Video, and you use associate on the relationship to now associate it to a Post, the owner key is incorrect.

This is because this line...
https://github.com/laravel/framework/blob/5.8/src/Illuminate/Database/Eloquent/Concerns/HasRelationships.php#L236-L238

...passes through the owner key based on the model it is currently associated to. Before your change this didn't make any difference because the key name when associating the relationship was always taken from the model that is being passed in to associate().

So I believe your change is breaking without any changes to the HasRelationships method.

[staudenmeir]

Thanks for reporting. Reverted in #29802.

There's apparently no other way to fix the bug.

[lindyhopchris]

Ah great, thanks for reverting. It's a shame there's no other way to fix it, as your change made sense... as long as both morph types had the same custom owner key.