[5.6] Add withDefault() support to MorphTo relationships

[staudenmeir]

Adds withDefault() support to MorphTo relationships:

public function commentable()
{
    return $this->morphTo()->withDefault();
}

The tests are taken from DatabaseEloquentBelongsToTest.

Fixes #24055.

[vpratfr]

Hi,

I am the person who originially opened #24055.

I fail to understand how calling ->withDefault() could determine how to create the related model instance.

According to me, this should throw an exception because the Comment class has no idea what to create as default (a video or a post?)

So the only valid use case of withDefault for MorphTo is when you provide a dynamic callback.

For instance:

FooModality extends Model

BarModality extends Model

Mission extends Model  
    has attributes : 'template', 'modality_type', 'modality_id'

    public function modality() : MorphTo {
        return $this->morphTo('modality')
                ->withDefault(function() {
                    // Eloquent needs to update our 'modality_type', 'modality_id' columns
                    if ($this->template==='foo') return new FooModality();
                    if ($this->template==='bar') return new BarModality();

                    throw new IllegalArgumentException("Mission type is not handled");
                });
    }

If we want to be able to use withDefault() then we need to introduce somehow a mecanism to decide which class of related model to instanciate.