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Overview

By default, when specifying an Filter::relation() or Filter::morphRelation(), fields within that relationship are not included in the allowed filter list. You can specify allowed filters inside a relation in two ways.

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includeRelationFields()

Use ->includeRelationFields() on Filter::relation() or Filter::morphRelation().
This method instructs the package to look for AllowedField filters within the allowedFilters() method of the relation model.
The relationship method MUST have return type specified, and the related model MUST also implement IsFilterable.
public function allowedFilters(): AllowedFilterList
{
    return Filter::only(
        Filter::relation('manufacturer', [FilterType::HAS])
            ->includeRelationFields()
    );
}
For Filter::morphRelation(), you should specify the models for which to include the relation fields for. 
public function allowedFilters(): AllowedFilterList
{
    return Filter::only(
        Filter::morphRelation(
            'subscribable',
            [FilterType::HAS_MORPH],
        )->includeRelationFields([
            FoodDeliveryService::class,
            Saas::class,
        ])
    );
}

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Define allowedFilters

Alternatively, if you don’t want to use ->includeRelationFields(), you can define allowedFilters for each Filter::relation() and Filter::morphType(). 
public function allowedFilters(): AllowedFilterList
{
    return Filter::only(
        Filter::relation(
            target: 'manufacturer',
            types: [FilterType::HAS],
            allowedFilters: Filter::only(
                Filter::field('name', [FilterType::LIKE])
            )
        )
    );
}

public function allowedFilters(): AllowedFilterList
{
    return Filter::only(
        Filter::morphRelation('subscribable', [FilterType::HAS_MORPH],
            Filter::morphType(
                type: FoodDeliveryService::class,
                allowedFilters: Filter::only(
                    Filter::field('name', [FilterType::EQUAL])
                )
            ),
            Filter::morphType(
                type: Saas::class,
                allowedFilters: Filter::only(
                    Filter::field('name', [FilterType::EQUAL])
                )
            ),
        )
    );
}