How to filter by polymorphic resource association filter?

Inizia con la dichiarazione dell’associazione polimorfica documentable di DocumentsDocumentableResource e con i due esempi di richieste di filtro. Traccia il modo in cui JSONAPI Resources convalida i filtri per le relazioni polimorfiche e determina se è supportato il dispatch specifico per tipo. Il lavoro è considerato completato quando esiste un modo documentato e testato per invocare il filtro appropriato, oppure una spiegazione chiaramente circoscritta della limitazione.

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Description

Bug reports:

If for example I have such setup, I have Document model that have many DocumentsDocumentables, which have documentable as polymorphic association and can have either Employee or Comment in it. I set up :

class Document < ApplicationRecord
  has_many :documents_documentables, dependent: :destroy
end

class DocumentsDocumentable < ApplicationRecord
  belongs_to :document
  belongs_to :documentable, polymorphic: true
end

class Employee < ApplicationRecord
  has_many :documents_documentables, as: :documentable
end

class Comment < ApplicationRecord
  has_many :documents_documentables, as: :documentable
end

module Api
  module V1
    class DocumentResource < BaseResource
      attributes :file_name

      has_many :documents_documentables

      filter :id
    end
  end
end

module Api
  module V1
    class DocumentsDocumentableResource < BaseResource
      attributes :documentable_id, :documentable_type, :document_id

      has_one :document
      has_one :documentable, polymorphic: true, eager_load_on_include: false
    end
  end
end

module Api
  module V1
    class EmployeeResource < BaseResource
      filter :custom_employee_filter, apply: lambda { |records, value, _options|
                                  puts "custom employee filter"
                                }
    end
  end
end

module Api
  module V1
    class CommentResource < BaseResource
      filter :custom_comment_filter, apply: lambda { |records, value, _options|
                                  puts "custom comment filter"
                                }
    end
  end
end

Having this setup, how can I filter DocumentDocumentable by documentable.custom_<employee | comment>_filter then?

I tried smth like that:

/api/v1/documents-documentables?filter=documentable.customEmployeeFilter=filterValue , or
/api/v1/documents-documentables?filter=documentable.customCommentFilter=filterValue

Both gives me error:

{
  error: {
    code: "102",
    detail: "documentable.customEmployeeFilter is not allowed.",
    status: "400",
    title: "Filter not allowed"
  }
}

Is it possible at all somehow to call both filters, and if yes - how should I change my resources, is there any param I can pass to further specialize polymorpthic resource's type?