How to delete polymorphic models cascade

If you use a polymorphic model in your Rails application, like in example

class Trade < ActiveRecord::Base
  has_many :gl_entries, as: :source, dependent: :destroy
end

class GlEntry < ActiveRecord::Base
  belongs_to :source, polymorphic: true
end

You will not be able to add the usual foreign keys for cascading delete records. But this can be implemented using a database.

To do this, you need to write a trigger in the database that will run the delete function for each record.

CREATE FUNCTION deleteGlEntriesOfTrade()
  RETURNS TRIGGER
  SET SCHEMA 'public'
  LANGUAGE plpgsql 
  AS $$
  BEGIN
    DELETE FROM gl_entries WHERE source_id = OLD.id AND source_type = 'Trade';
    RETURN OLD;   
  END;
  $$;

CREATE TRIGGER deleteTradesGlEntriesTrigger 
  BEFORE DELETE ON trades
  FOR EACH ROW EXECUTE PROCEDURE deleteGlEntriesOfTrade();

Create a migration and use :)