Polymorphic Relationships Not Found On Aliased Resource

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• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• [] The feature I'm asking for is compliant with the JSON:API spec.

Description

We are using the v0-11-dev at risk.

If we define all that is necessary for a polymorphic relationship, we can include the related resources via something like GET /people/123?include=pets. Example code that works:

class PersonResource < JSONAPI::Resource
  relationship :pets, polymorphic: true, to: :many, polymorphic_types: %w[Cat Dog]
end

However if we "alias" the primary resource, the analogous request (GET /pet_owners/123?include=pets) does not find the related resources when we include the relationship:

  class PetOwnerResource < JSONAPI::Resource
    model_name 'Person'

    relationship :pets, polymorphic: true, to: :many, polymorphic_types: %w[Cat Dog]
end

It seems the related resource lookup is not taking into account that the primary/source resource might be aliased and using model_name.

Expected: the related resources are found.

No source file or test is named. Start by reproducing the aliased PetOwnerResource example with GET /pet_owners/123?include=pets, then compare it with the working PersonResource request and trace polymorphic relationship lookup around model_name. Done means the aliased resource finds and includes the Cat and Dog relationships as expected.