model.collection_model_ids (many to many)

Please go to http://rails-bestpractices.com/posts/8-model-collection_model_ids-many-to-many

Before:

class User < ActiveRecord::Base
  has_many :user_role_relationships
  has_many :roles, :through => :user_role_relationships
end

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

class Role < ActiveRecord::Base
end

<% form_for @user do |f| %>
  <%= f.text_field :email %>
  <% Role.all.each |role| %>
    <%= check_box_tag 'role_id[]', role.id, @user.roles.include?(role) %>
    <%= role.name %>
  <% end %>
<% end %>

class UsersController < ApplicationController

  def update
    @user = User.find(params[:id])
    if @user.update_attributes(params[:user])
      @user.roles.delete_all
      (params[:role_id] || []).each { |i| @user.roles << Role.find(i) }
    end
  end

end

After:

<% form_for @user do |f| %>
  <% Role.all.each |role| %>
    <%= check_box_tag 'user[role_ids][]', role.id, @user.roles.include?(role) %>
    <%= role.name %>
  <% end %>
  <%= hidden_field_tag 'user[role_ids][]', '' %>
<% end %>

class UsersController < ApplicationController

  def update
    @user = User.find(params[:id])
    @user.update_attributes(params[:user])
    # the same as @user.role_ids = params[:user][:role_ids]
  end

end