[Rails] Not work has_many :through association

Not work has_many :through association in rails 2.3.5

Models:

class Role < ActiveRecord::Base
attr_accessible :name

has_many :assignments
has_many :users, :through => :assignments
end

class User < ActiveRecord::Base
attr_accessible :name

has_many :assignments
has_many :roles, :through => :assignments
end

class Assignment < ActiveRecord::Base
belongs_to :user
belongs_to :role
end

In the view form of user

- for user in User.all
= check_box_tag 'role[user_ids][]', user.id, @role.users.include?
(user)
=h user.name

When I create a new user, nothing happens, the array of user_ids is
empty

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