usersテーブル

Column	Type	Options
name	string	null: false, index: true, unique: true

Association

-has_many :messages -has_many :group_users -has_many :groups, through :group_users

massageテーブル

Column	Type	Options
user	reference	null: false, foreign_key: true
group	reference	null: false, foreign_key: true
comment	text	null: false
image	string	null: false

Association

-belongs_to :user -belongs_to :group

groupsテーブル

Column	Type	Options
groupname	string	null: false, unique: true

Association

-has_many :group_users -has_many :messages -has_many :users, through :group_users

group_usersテーブル

Column	Type	Options
user	reference	null: false, foreign_key: true
group	reference	null: false, foreign_key: true

Association

-belongs_to :user -belongs_to :group