Usersテーブル

column	type	option
id	integer	null false
name	string	null false ※add_index
email	string	null false
password	string	null false
confirm password	string	null false
group_id	integer	null false

association

has_many :messages
has_many :groups_users
has_many :groups, through: :groups_users

Massagesテーブル

column	type	option
id	integer	null false
body	text
image	string
group_id	integer	null false
user_id	integer	null false

association

belongs_to :user
belongs_to :group

Groupsテーブル

column	type	option
id	integer	null false
name	string	null false ※add_index

association

has_many :messages
has_many :groups_users
has_many :users, through: :groups_users

groups_usersテーブル

column	type	option
id	integer	null false
user_id	integer	null false
group_id	integer	null false

association

belongs_to :user
belongs_to :group