Active Admin belongs to filter with custom foreign key

module ActiveAdmin
  module Filters
    # This form builder defines methods to build filter forms such
    # as the one found in the sidebar of the index page of a standard resource.
    class FormBuilder < ::ActiveAdmin::FormBuilder
      include ::ActiveAdmin::Filters::FormtasticAddons
      self.input_namespaces = [::Object, ::ActiveAdmin::Inputs::Filters, ::ActiveAdmin::Inputs, ::Formtastic::Inputs]

      # TODO: remove input class finders after formtastic 4 (where it will be default)
      self.input_class_finder = ::Formtastic::InputClassFinder

      def filter(method, options = {})
        if method.present? && options[:as] ||= default_input_type(method)

          if __klass = self.object.klass
            __reflections = __klass.reflections.keys
            if __reflections.include?(method)
              __assoc_type = __klass.reflect_on_association(method).try(:macro)
              if __assoc_type && __assoc_type == :belongs_to
                if method.to_s.classify.safe_constantize
                  _collection_class = method.to_s.classify.safe_constantize
                  options[:collection] ||= _collection_class.send(:all).map {|c| [c.id, c.display_name]}
                end
                method = __klass.reflections[method].foreign_key
              end
            end
          end
          template.concat input(method, options)
        end
      end

      protected

    end

  end
end