[6.x] Add AsMorphPivot trait

[idsulik]

The benefit of the trait is that you can add it to your custom model(which extends base Model, but not Pivot or MorphPivot model).
There is already exists AsPivot trait which is useful, so I add AsMorphPivot for morph pivot models

[taylorotwell]

Why would you have a morph pivot model that does not extend MorphPivot?

[idsulik]

@taylorotwell , because of the error - Invalid parameter number: parameter was not defined (SQL: update "model_properties" set "status" = rejected, "updated_at" = 2020-09-23 12:48:00 where "" = section and "" = 469)

$sectionProperty = factory(ModelProperty::class)->create([
  'relation_type' => ModelProperty::RELATION_TYPE_SECTION,
  'relation_id' => $section->id,
  'status' => ModelProperty::STATUS_DRAFT,
]);
$sectionProperty->status = $newStatus;
$sectionProperty->save();

It works only if I extends base Model and use AsMorphPivot trait

[idsulik]

@taylorotwell One more reason #25851 to use it on model which extends my own abstract model

[idsulik]

If extends MorphPivot and try to update the model I get the sql:
update "model_properties" set "status" = ?, "updated_at" = ? where "" = ? and "" = ? - where are morph column names?

ModelProperty is simple MorphPivot model:

It works only if I add the AsMorphPivot trait

[taylorotwell]

We aren't adding new features to 6.x and this also needs a lot more explanation. It feels like we should fix the root cause of your problem - not just slap everything in a trait and say you need to extend Model.