HasMany field used for a relation (but not an association)

[such]

• What were you trying to do?

I've updated administrate to master and following #1633 some of my dashboards are broken.
We are (ab)using the HasMany field for displaying relations that are not associations:

class Customer
  def fellow_customers_that_are_hard_to_represent_with_an_association
    Customer.scope.where...
  end
end

class CustomerDashboard < Administrate::BaseDashboard
  ATTRIBUTE_TYPES = {
    fellow_customer: Field::HasMany.with_options(class_name: 'Customer'),
  }
end

• What did you end up with (logs, or, even better, example apps are great!)?

ActionView::Template::Error:
       undefined method `class_name' for nil:NilClass
       Did you mean?  class_eval
     # ./vendor/ruby/2.7.0/bundler/gems/administrate-0f8624ca1d24/lib/administrate/field/associative.rb:15:in `associated_class_name'
     # ./vendor/ruby/2.7.0/bundler/gems/administrate-0f8624ca1d24/lib/administrate/field/associative.rb:44:in `associated_class_name'
     # ./vendor/ruby/2.7.0/bundler/gems/administrate-0f8624ca1d24/lib/administrate/field/associative.rb:37:in `associated_dashboard'
     # ./vendor/ruby/2.7.0/bundler/gems/administrate-0f8624ca1d24/lib/administrate/field/has_many.rb:82:in `includes'
     # ./vendor/ruby/2.7.0/bundler/gems/administrate-0f8624ca1d24/lib/administrate/field/has_many.rb:57:in `resources'

• What versions are you running?
  • Rails: 6.0.3.2
  • administrate: master

This pattern is still useful I think. I've tried converting those methods to associations and it's not always possible.
Can the behaviour of the former HasMany be restored in a new Field type maybe?

[pablobm]

Uh, that's an interesting use case; thank you for letting us know. My initial thought is that it could be solved with a new field type, although I'm not 100% sure. Would you be able to give it a go?