MorphToMany/MorphedByMany field not filling withPivotValue defined on relation

[dmason30]

• Laravel Version: 10.x
• Nova Version: 4.23.0

Description:

Same issue as #3639 which was eventually implemented for BelongsToMany.

Say we have a Music model with a contributables pivot which can morph to either Singer or Producer models.
The pivot also has an example field as defined below:

Schema::create('contributables', function (Blueprint $table) {
    $table->foreignIdFor(Music::class)->constrained()->cascadeOnDelete();
    $table->nullableMorphs('contributable');
    $table->string('example');
});

app/Models/Music.php

public function singers()
{
    return $this->morphedByMany(Singer::class, 'contributables')
        ->withPivotValue('example', 'some singer value');
}

public function producers()
{
    return $this->morphedByMany(Producer::class, 'contributables')
        ->withPivotValue('example', 'some producer value');
}

When I run the below code

$model = Music::find(1);
$model->singers()->attach(Singer::find(1));
$model->producers()->attach(Producer::find(1))

New record added to contributables table:

music_id	contributable_type	contributable_id	example
1	singers	1	some singer value
1	producers	1	some producer value

However, when doing this in nova the some singer value and some producer value are not inserted:

app/Nova/MusicResource.php

MorphedByMany::make('Singers', 'singers', SingerResource::class),
MorphedByMany::make('Producers', 'producers', ProducerResource::class),

Because example is not nullable it will give the following error when trying to attach:

SQLSTATE[HY000]: General error: 1364 Field 'example' doesn't have a default value (SQL: insert into `contributables` (`music_id`, `contributable_type`, `contributable_id`) values (1, 'singers', 1))

Workaround:

Define hidden fields with default values on the field

app/Nova/MusicResource.php

MorphedByMany::make('Singers', 'singers', SingerResource::class)->fields(fn () => [
   Hidden::make('example')->default(fn () => 'some singer value')
]),

Fix

MorphedResourceAttachController needs to have an equivalent of the Laravel\Nova\Query\Mixin\BelongsToMany::getDefaultPivotAttributes() method that the ResourceAttachController uses to apply the default values.