Linked data with polymorphism does not work with custom `key_type`

lib/jsonapi/resource_serializer.rb:464 から始めて foreign_key_value を調べ、カスタム key_type が設定されている場合にポリモーフィックなリレーションがどのように ID を取得するかに注目してください。報告されたケースを再現し、シリアライズされたリレーションの ID が常に直接の関連付けの ID を使うのではなく、カスタム key_type に従うことを示すカバレッジを追加してください。

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• Feature request.
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Description

If you create a polymorphic association with custom key_type id is always taken which leads to a wrong data sharing.

Bug reports:

Everything is on lib/jsonapi/resource_serializer.rb:464
When calling source.public_send("#{relationship.name}_id"), it is fine (faster) but it does not take care about custom key_type.
But if you call source.public_send(relationship.name).try(:id), you're using the custom key_type.

Features:

We can set another argument to has_one to be able to mention that the key_type has been modified.
Here is my proposition:

# app/resources/my_awesome_resource
has_one :positionable, polymorphic: true, foreign_key: :positionable_id, foreign_key_type_changed: true

# jsonapi/resource_serializer.rb

  def foreign_key_value(source, relationship)
      # If you have changed the key_name, don't even try to look at `"#{relationship.name}_id"` 
      # just load the association and call the custom key_name
      foreign_key_type_changed = relationship.options[:foreign_key_type_changed] || false
      related_resource_id =
        if source.preloaded_fragments.has_key?(format_key(relationship.name))
          source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
        elsif !foreign_key_type_changed && source.respond_to?("#{relationship.name}_id")
          # If you have direct access to the underlying id, you don't have to load the relationship
          # which can save quite a lot of time when loading a lot of data.
          # This does not apply to e.g. has_one :through relationships.
          source.public_send("#{relationship.name}_id")
        else
          source.public_send(relationship.name).try(:id)
        end
      return nil unless related_resource_id
      @id_formatter.format(related_resource_id)
    end