gistfile1.txt

mysql> describe contacts;
+------------+---------------------+------+-----+---------+----------------+
| Field      | Type                | Null | Key | Default | Extra          |
+------------+---------------------+------+-----+---------+----------------+
| id         | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment | 
| user_id    | bigint(20) unsigned | NO   |     | NULL    |                | 
| first_name | varchar(255)        | YES  |     | NULL    |                | 
| last_name  | varchar(255)        | YES  |     | NULL    |                | 
| company    | varchar(255)        | YES  |     | NULL    |                | 
| address_1  | varchar(255)        | YES  |     | NULL    |                | 
| address_2  | varchar(255)        | YES  |     | NULL    |                | 
| city       | varchar(255)        | YES  |     | NULL    |                | 
| state      | char(2)             | YES  |     | NULL    |                | 
| zip        | tinyint(5) unsigned | YES  |     | NULL    |                | 
| home_phone | bigint(11) unsigned | YES  | MUL | NULL    |                | 
| cell_phone | bigint(11) unsigned | YES  | MUL | NULL    |                | 
| created_at | datetime            | YES  |     | NULL    |                | 
| updated_at | datetime            | YES  |     | NULL    |                | 
+------------+---------------------+------+-----+---------+----------------+
14 rows in set (0.01 sec)

mysql> describe contact_groups;
+------------+---------------------+------+-----+---------+----------------+
| Field      | Type                | Null | Key | Default | Extra          |
+------------+---------------------+------+-----+---------+----------------+
| id         | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment | 
| user_id    | bigint(20) unsigned | NO   | MUL | NULL    |                | 
| name       | varchar(255)        | NO   |     | NULL    |                | 
| created_at | datetime            | YES  |     | NULL    |                | 
| updated_at | datetime            | YES  |     | NULL    |                | 
+------------+---------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> describe contact_group_members;
+------------------+---------------------+------+-----+---------+----------------+
| Field            | Type                | Null | Key | Default | Extra          |
+------------------+---------------------+------+-----+---------+----------------+
| id               | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment | 
| contact_id       | bigint(20) unsigned | NO   | MUL | NULL    |                | 
| contact_group_id | bigint(20) unsigned | NO   |     | NULL    |                | 
+------------------+---------------------+------+-----+---------+----------------+

gistfile2.rb

class User < ActiveRecord::Base
  has_many :contacts
  has_many :contact_groups
end

class Contact < ActiveRecord::Base
  belongs_to :user
  has_many :contact_group_members
  has_many :contact_groups, :through => :contact_group_members
end

class ContactGroup < ActiveRecord::Base
  belongs_to :user
  has_many :contact_group_members
  has_many :contacts, :through => :contact_group_members
end

class ContactGroupMember < ActiveRecord::Base
  belongs_to :contacts
  belongs_to :contact_groups
end