users              groups        groups_users

表groups_users只存在表结构，没有相应的model,controller,views等

表groups_users的生成（该表没有id）

  def change
    create_table(:groups_users, :id => false) do |t|
      t.integer :group_id
      t.integer :user_id

      t.timestamps
    end
  end

model中

class Group < ActiveRecord::Base
  has_and_belongs_to_many :users
  attr_accessor :user_id
end


class User < ActiveRecord::Base
  has_and_belongs_to_many :groups
end

 groups的show（haml）

= semantic_attributes_for @group do |attr|
  = attr.attributes do
    = attr.attribute :name
    = attr.attribute :description

  = attr.attributes "group_users" do
    - if @group.users
      - @group.users.each do |u|
        = attr.attribute "user:", :value => u.name
        = button_to "删除组员", { :action => "delete_groupmember", :id => @group.id, :user_id => u.id,
          :confirm => "您确定要删除吗?", :method => :delete}

= semantic_form_for(@group, :url => {:action => "add_user_to_group"}, :html => {:method => :post}) do |f|
  = f.inputs do
    = f.hidden_field :id
    = f.input :user_id, :collection => User.all
  = f.buttons do
    = f.commit_button

 groups中的controller

  def add_user_to_group
    @group = Group.find(params[:group][:id])
    
    unless params[:group][:user_id].blank?
      @user = User.find(params[:group][:user_id])
      @group.users << @user unless @group.users.any?{ |u| u == @user }
      redirect_to @group, :notice => I18n.t("flash.groups.grouping_success.notice")
    else
      redirect_to @group, :notice => I18n.t("flash.groups.grouping_failed.notice")
    end
  end

 方法push_with_attributes( )和<<方法的作用一样，都是给两个Model之间设置连接

  def delete_groupmember
    @group = Group.find(params[:id])
    @user = User.find(params[:user_id])
    @group.users.delete(@user)
    redirect_to @group, :notice => I18n.t("flash.groups.delete_groupmember.notice")
  end

或者通过checkboxes来实现HABTM关系

<% for group in Group.find(:all) %>  
    <div>  
      <%= check_box_tag "user[group_ids][]", category.id, @uer.groups.include?(group) %>  
      <%= group.name %>  
    </div>  
<% end %>  

controller中

params[:user][:group_ids] ||= [] 

 具体请见：

http://asciicasts.com/episodes/17-habtm-checkboxes

http://asciicasts.com/episodes/258-token-fields