Debugging for github.com/ernie/ransack/issues/173 and github.com/gregbell/active_admin/pull/1979

ransack_form.rb

gem 'rails', '3.2.13'
gem 'ransack'
require 'rails/all'
require 'ransack'
 
ActiveRecord::Base.establish_connection adapter:  'sqlite3',
                                        database: ':memory:'
 
ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.integer :id
  end
 
  create_table :comments, force: true do |t|
    t.integer :id
    t.integer :commentable_id
    t.string  :commentable_type
  end
end
 
class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true
end
 
class User < ActiveRecord::Base
  has_many :comments, as: :commentable
end
 
# ----------------------------------------------
 
# this works
Comment.search(commentable_type_cont:'e').result

# But when building a form:
form_for the_current_ransack_search_object do |f|
  f.input :commentable_type
end
# `ActionView::Helpers::InstanceTag#value_before_type_cast` calls this:
object.respond_to? "commentable_type_cont_before_type_cast"
# and `Ransack::Search#respond_to?` calls this:
base.attribute_method? "commentable_type_cont_before_type_cast"
# which ends up calling `.klass` on the polymorphic association relfection,
# which returns an incorrect object.