Search in nested polymorphic association attribute

[esti]

Hello,

I'm stuck with defining a search field on an attribute that is in a second level polymorphic association. Let me explain:

Given these models

class UserCase < ApplicationRecord
  delegated_type :user_caseable, types: %w( WorkCase HobbyCase ), dependent: :destroy
end

class WorkCase < ApplicationRecord
  delegated_type :work_caseable, types: %w( CaretakerCase OtherWorkerCase ), dependent: :destroy
end

class CaretakerCase < ApplicationRecord
  # has a column named work_modality
end

I want to perform a search of user cases where they work_modality column of CartakerCase equals "intern".

What should the form field be named?

I tried this

In my view:

<%= search_form_for @q, url: search_path, html: {method: :get} %>
  <%= f.search_field :user_caseable_of_WorkCase_type_and_work_caseable_of_CaretakerCase_type_work_modality_eq %>
<% end %>

In my controller:

@user_cases = UserCase.ransack(params[:q]).result

And I get this error when rendering the view:
"Polymorphic associations do not support computing the class"

What is the correct name for the field?

Thanks in advance