users
id: int // users has_many chat_users

chats
id: int // chats has_many chat_users

chat_users
id: int
chat_id: int (foreign key) // chat_users belongs_to chat
user_id: int (foreign key) // chat_users belongs_to user

users, id: 1
users, id: 2

chats, id: 1 // <---------
chats, id: 2

chat_users, id: 1, chat_id: 1, user_id: 1 // <-------
chat_users, id: 2, chat_id: 1, user_id: 2 // <-------
chat_users, id: 3, chat_id: 2, user_id: 2