has_many :through habtm

Here is my setup:

class Product < ActiveRecord::Base
  belongs_to :catalog
  has_and_belongs_to_many :product_groups
end

class ProductGroup < ActiveRecord::Base
  belongs_to :catalog
  has_and_belongs_to_many :products
end

class Catalog < ActiveRecord::Base
  has_many :product_groups
  has_many :products, :through => :product_groups
end

I get:
Invalid source reflection macro :has_and_belongs_to_many for
has_many :products, :through => :product_groups. Use :source to
specify the source reflection.

As written, there's no direct way to do this association in Rails. But
wouldn't it be sufficient to use a plain has_many :products in
Catalog? You've got a corresponding belongs_to in Product...

--Matt Jones

yeah, I think it is sufficient. thanks.