When transferring this into real code, we first generate the Chat model. The problem here is that we have a many-to-many relationship between users and chats. In MySQL, this kind of relationship requires a table, to store the relations between all entities separately.

Those tables are called join tables. Instead of using a foreign key on the chat or a user to save the relationship, we have a table called user_chats. The user's ID and the chat's ID are associated with each other inside of this table. If a user participates in multiple chats, they will have multiple rows in this table, with different chat IDs.