How to set value from url params without specifyng it in the request body?

Questions & Help Questions
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1

I’m working with Ash and AshJsonApi and encountering an issue with how article_id is being handled in the URL parameters for my comment resource.

  actions do
    create :add_comment do
      accept [:comment, :article_id]

      change set_attribute(:article_id, arg(:article_id))
  end

My domain:

  json_api do
    routes do
      base_route "/articles", Article do
          base_route "/:article_id/add_comment", Comment do
            post :add_comment
        end
      end
    end
  end
end

I intended to use change set_attribute(:article_id, arg(:article_id)) in the add_comment action of the comment resource to automatically set the article_id from the URL parameters, without having to include it in the request body. However, I’m encountering an error where it says invalid_body and required properties are missing: channel_id

How can I configure Ash to use the article_id from the URL parameters for the add_comment action?