has many polymorphic images through another polymorphic model

Question

Greets to ruby developers.

I'm stuck with ActiveRecord model associations. I illustrate what I want to do by a tables. I have these tables:

Products:
+----+-----------+
| id | name      |
+----+-----------+
|  1 | Cellphone |
+----+-----------+

Images:
+----+-------+--------------+----------------+
| id | url   | imageable_id | imageable_type |
+----+-------+--------------+----------------+
|  1 | 1.jpg |            1 | Product        |
|  2 | 2.jpg |            1 | Product        |
|  3 | 3.jpg |          777 | User           | # User for example
+----+-------+--------------+----------------+

Photos:
+----+----------+--------------+----------------+-------+
| id | image_id | photoable_id | photoable_type | cover |
+----+----------+--------------+----------------+-------+
|  1 |        1 |            1 | Product        |     0 |
|  2 |        2 |            1 | Product        |     1 |
|  3 |       23 |            7 | Service        |     1 | # Service for example
+----+----------+--------------+----------------+-------+

Any model can have many images (e.g. User has many avatars). Product can have many images, but one of these images must be a product cover (a main image). So I add a Photo model with 'cover' flag. Photo must be associated with Image.

Models:

class Product < ActiveRecord::Base
  # need assocs
end
class Image < ActiveRecord::Base
  # need assocs
end
class Photo < ActiveRecord::Base
  # need assocs
end

I want to get something like this:

p = Product.find(1)
p.images.each do |image|
  # As you can see, cover attribute included to result set
  puts "Image url: #{image.url}, Is cover: #{image.cover ? 'yes' : 'no'}"
end

Please help me with associations between models