ActiveRecord doesnt like foreign keys?

gistfile1.txt

class Category < ActiveRecord::Base
  has_many :assignments, :foreign_key => 'category_uuid', :primary_key => 'uuid'
end

class Post < ActiveRecord::Base
  has_many :assignments, :foreign_key => 'post_uuid', :primary_key => 'uuid'
end

class Assignment < ActiveRecord::Base
  belongs_to :post, :foreign_key => 'uuid', :primary_key => 'post_uuid'
  belongs_to :category, :foreign_key => 'uuid', :primary_key => 'category_uuid'
end




Example console session:

>> p = Post.create
  Post Create (0.5ms)   INSERT INTO "posts" ("name", "created_at", "uuid", "updated_at") VALUES(NULL, '2010-02-03 00:09:29', NULL, '2010-02-03 00:09:29')
=> #<Post id: 1, uuid: nil, name: nil, created_at: "2010-02-03 00:09:29", updated_at: "2010-02-03 00:09:29">
>> p.uuid = 'abc'
=> "abc"
>> p.save
  Post Update (0.8ms)   UPDATE "posts" SET "updated_at" = '2010-02-03 00:09:35', "uuid" = 'abc' WHERE "id" = 1
=> true
>> c = Category.create
  Category Create (0.7ms)   INSERT INTO "categories" ("name", "created_at", "uuid", "updated_at") VALUES(NULL, '2010-02-03 00:09:40', NULL, '2010-02-03 00:09:40')
=> #<Category id: 1, name: nil, uuid: nil, created_at: "2010-02-03 00:09:40", updated_at: "2010-02-03 00:09:40">
>> c.uuid = 'def'
=> "def"
>> c.save
  Category Update (0.7ms)   UPDATE "categories" SET "updated_at" = '2010-02-03 00:09:49', "uuid" = 'def' WHERE "id" = 1
=> true
>> a = Assignment.create
  Assignment Create (0.4ms)   INSERT INTO "assignments" ("created_at", "updated_at", "post_uuid", "category_uuid") VALUES('2010-02-03 00:09:53', '2010-02-03 00:09:53', NULL, NULL)
=> #<Assignment id: 1, post_uuid: nil, category_uuid: nil, created_at: "2010-02-03 00:09:53", updated_at: "2010-02-03 00:09:53">
>> a.post_uuid = 'abc'
=> "abc"
>> a.category_uuid = 'def'
=> "def"
>> a.save
  Assignment Update (0.7ms)   UPDATE "assignments" SET "updated_at" = '2010-02-03 00:10:06', "post_uuid" = 'abc', "category_uuid" = 'def' WHERE "id" = 1
=> true
>> a.reload
  Assignment Load (0.4ms)   SELECT * FROM "assignments" WHERE ("assignments"."id" = 1) 
=> #<Assignment id: 1, post_uuid: "abc", category_uuid: "def", created_at: "2010-02-03 00:09:53", updated_at: "2010-02-03 00:10:06">
>> a.post
=> nil