I have a case of polymorphic association and STI here.
# app/models/car.rb
class Car < ActiveRecord::Base
belongs_to :borrowable, :polymorphic => true
end
# app/models/staff.rb
class Staff < ActiveRecord::Base
has_one :car, :as => :borrowable, :dependent => :destroy
end
# app/models/guard.rb
class Guard < Staff
end
In order for the polymorphic assocation to work, according to the API documentation on Polymorphic Assocation, http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#label-Polymorphic+Associations that I have to set borrowable_type
to the base_class
of STI models, that is in my case is Staff
.
The question is: Why doesn't it work if the borrowable_type
set to STI class?
Some test to prove it:
# now the test speaks only truth
# test/fixtures/cars.yml
one:
name: Enzo
borrowable: staff (Staff)
two:
name: Mustang
borrowable: guard (Guard)
# test/fixtures/staffs.yml
staff:
name: Jullia Gillard
guard:
name: Joni Bravo
type: Guard
# test/units/car_test.rb
require 'test_helper'
class CarTest < ActiveSupport::TestCase
setup do
@staff = staffs(:staff)
@guard = staffs(:guard)
end
test "should be destroyed if an associated staff is destroyed" do
assert_difference('Car.count', -1) do
@staff.destroy
end
end
test "should be destroyed if an associated guard is destroyed" do
assert_difference('Car.count', -1) do
@guard.destroy
end
end
end
But it seems to be true only with Staff instance. The results are:
# Running tests:
F.
Finished tests in 0.146657s, 13.6373 tests/s, 13.6373 assertions/s.
1) Failure:
test_should_be_destroyed_if_an_associated_guard_is_destroyed(CarTest) [/private/tmp/guineapig/test/unit/car_test.rb:16]:
"Car.count" didn't change by -1.
<1> expected but was
<2>.
Thanks
Good question. I had exactly the same problem using Rails 3.1. Looks like you can not do this, because it does not work. Probably it is an intended behavior. Apparently, using polymorphic associations in combination with Single Table Inheritance (STI) in Rails is a bit complicated.
The current Rails documentation for Rails 3.2 gives this advice for combining polymorphic associations and STI:
Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association.
In your case the base model would be "Staff", i.e. "borrowable_type" should be "Staff" for all items, not "Guard". It is possible to make the derived class appear as the base class by using "becomes" : guard.becomes(Staff)
. One could set the column "borrowable_type" directly to the base class "Staff", or as the Rails Documentation suggests, convert it automatically using
class Car < ActiveRecord::Base
..
def borrowable_type=(sType)
super(sType.to_s.classify.constantize.base_class.to_s)
end