Rails Group/User relationship using join table - has_many :users, :through => :members

add_members_uniqueness_index.rb

#############################################################################
# Migration file to create index so you don't get duplicate users for a group
#############################################################################
class AddMembersUniquenessIndex < ActiveRecord::Migration
  def self.up
  	add_index :group_members, [:group_id,:user_id], :unique => true
  end

  def self.down
 	remove_index :group_members, [:group_id,:user_id]
   end
end

gistfile2.txt

#  Users and Groups that were previously manually created.....
# Here's a dump of them:
ruby-1.9.2-p0 > Group.all
 => [#<Group id: 1, name: "Group 1", description: "Group One", created_at: "2011-02-27 18:55:01", updated_at: "2011-02-27 18:55:01">,
     #<Group id: 2, name: "Group 2", description: "Group Two", created_at: "2011-02-27 18:55:39", updated_at: "2011-02-27 18:55:39">] 

ruby-1.9.2-p0 > User.all
 => [#<User id: 1, name: "User 1", email: "user1@test.com", created_at: "2011-02-27 18:39:58", updated_at: "2011-02-27 18:39:58">,
     #<User id: 2, name: "User 2", email: "user2@test.com", created_at: "2011-02-27 18:40:20", updated_at: "2011-02-27 18:40:20">] 

# Get the first Group
ruby-1.9.2-p0 > g = Group.first
 => #<Group id: 1, name: "Group 1", description: "Group One", created_at: "2011-02-27 18:55:01", updated_at: "2011-02-27 18:55:01"> 
# List its current Users which is none
ruby-1.9.2-p0 > g.users
 => [] 



#################################################################################################################### 
# Add association of existing Group and User through the Group members join table 
#################################################################################################################### 
ruby-1.9.2-p0 > u = g.members.build(:user_id => 1)
 => #<Member id: nil, user_id: 1, group_id: 1, created_at: nil, updated_at: nil> 
# Save the Member
ruby-1.9.2-p0 > u.save
 => true 
  
# Reload the Group  
ruby-1.9.2-p0 > g = Group.first
 => #<Group id: 1, name: "Group 1", description: "Group One", created_at: "2011-02-27 18:55:01", updated_at: "2011-02-27 18:55:01"> 
# List its current Users which is one
ruby-1.9.2-p0 > g.users
 => [#<User id: 1, name: "User 1", email: "user1@test.com", created_at: "2011-02-27 18:39:58", updated_at: "2011-02-27 18:39:58">] 
  
  
#################################################################################################################### 
# Here's a way to create a new User and add to Group through the members join table. Note save not called   
#################################################################################################################### 
ruby-1.9.2-p0 > u = g.users.create(:name => "User 3", :email => "user3@test.com")
 => #<User id: 3, name: "User 3", email: "user3@test.com", created_at: "2011-02-27 20:35:37", updated_at: "2011-02-27 20:35:37"> 
ruby-1.9.2-p0 > g.users
 => [#<User id: 1, name: "User 1", email: "user1@test.com", created_at: "2011-02-27 18:39:58", updated_at: "2011-02-27 18:39:58">, #<User id: 3, name: "User 3", email: "user3@test.com", created_at: "2011-02-27 20:35:37", updated_at: "2011-02-27 20:35:37">] 
ruby-1.9.2-p0 > 
  
#################################################################################################################### 
# Create a new User and add to Group through the members join table. Note you must save the Group to update users and members tables  
#################################################################################################################### 
ruby-1.9.2-p0 > u = g.users.build(:name => "User 4", :email => "user4@test.com")
 => #<User id: nil, name: "User 4", email: "user4@test.com", created_at: nil, updated_at: nil> 
ruby-1.9.2-p0 > g.save
 => true 
  
####################################################################################################################  
# Destroy of User also removes Groups member relationship  (User - has_many :group_members, :dependent => :destroy)
#################################################################################################################### 
# Current list of Users for specified Group
ruby-1.9.2-p0 > g = Group.first
ruby-1.9.2-p0 > g.users
 => [#<User id: 1, name: "User 1", email: "user1@test.com", created_at: "2011-02-27 18:39:58", updated_at: "2011-02-27 18:39:58">,
     #<User id: 3, name: "User 3", email: "user3@test.com", created_at: "2011-02-27 20:35:37", updated_at: "2011-02-27 20:35:37">,
     #<User id: 4, name: "User 4", email: "user4@test.com", created_at: "2011-02-27 20:39:08", updated_at: "2011-02-27 20:39:08">] 
# Find and destroy user
ruby-1.9.2-p0 > u = User.find(4)
ruby-1.9.2-p0 > u.destroy
# View updated Group/User relationship
ruby-1.9.2-p0 > g = Group.first
 => #<Group id: 1, name: "Group 1", description: "Group One", created_at: "2011-02-27 18:55:01", updated_at: "2011-02-27 18:55:01"> 
ruby-1.9.2-p0 > g.users
 => [#<User id: 1, name: "User 1", email: "user1@test.com", created_at: "2011-02-27 18:39:58", updated_at: "2011-02-27 18:39:58">,
     #<User id: 3, name: "User 3", email: "user3@test.com", created_at: "2011-02-27 20:35:37", updated_at: "2011-02-27 20:35:37">] 

group.rb

# == Schema Information
#
# Table name: groups
#
#  id          :integer         not null, primary key
#  name        :string(255)
#  description :string(255)
#  created_at  :datetime
#  updated_at  :datetime
#
class Group < ActiveRecord::Base

  # destroy the relationship if group is deleted
 has_many :members, :dependent => :destroy

 # has many through members join table
 has_many :users, :through => :members
 
end

member.rb

# == Schema Information
#
# Table name: members
#
#  id         :integer         not null, primary key
#  user_id    :integer
#  group_id   :integer
#  created_at :datetime
#  updated_at :datetime
#
class Member < ActiveRecord::Base
   belongs_to :group
   belongs_to :user

   validates :user_id, :presence => true 
   validates :group_id, :presence => true	
   # avoid in memory duplicates. You will also need add unique index to db
   validates :user_id, :uniqueness => {:scope => [:user_id, :group_id]}	


end

user.rb

# == Schema Information
#
# Table name: users
#
#  id         :integer         not null, primary key
#  name       :string(255)
#  email      :string(255)
#  created_at :datetime
#  updated_at :datetime
#
class User < ActiveRecord::Base

    # destroy the user in the members list if the user is deleted
    has_many :members, :dependent => :destroy

    # has many through members join table
    has_many :groups, :through => :members

end