Spina Custom Part

_form.html.haml

-# app/views/spina/admin/parts/cards/_form.html.haml
.page-form-label= f.object.title
.page-form-control

# this line, as generated, produces "uninitialized constant Card"
  .select-dropdown= f.select :card_id, Card.all.pluck(:title, :id)
  
# this line, with the model class in full gets "ActionView::Template::Error (undefined method `all' for Spina::Parts::Card:Class)"
  .select-dropdown= f.select :card_id, Spina::Parts::Card.all.pluck(:title, :id)

card.rb

module Spina
  module Parts
    class Card < Base

      attr_json :card_id, :integer
      attr_json :card_title, Spina::Parts::Line.to_type
      attr_json :content, Spina::Parts::Text.to_type
      attr_json :image, Spina::Parts::Image.to_type
      attr_json :footer, Spina::Parts::Line.to_type

      def content
        Card.find(card_id)
      end

    end
  end
end