model.collection_model_ids (many-to-many)

Bad Smell

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

As you see, user and role models are many to many, and there is a form to associate one user to many roles. In the controller, we first remove all the roles of the user, then assign the roles to user according to the selection on checkbox. It's ugly to assign the roles to user by ourselves, we should use rails's facility.

Refactor

<% 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

What we do is changing the name of checkbox from "role_id[]" to "user[role_ids][]", so the role_ids will be automatically assigned to the user model. Please note that there is a hidden_field_tag 'user[role_ids][]', its value is empty, which promises that all the roles association to user will be destroyed if no user roles checkbox selected. Now our controller keeps simple again.