Many to Many relationship in Ecto

Question

I have a Users model and a Chats model. Intuitively multiple people will belong to the same chat group at any time and each person can have many chat groups. Therefore the chat group must belong to multiple user_id's.

My schema for the chat group and users are:

schema "chatGroups" do
    field :name, :string
    has_many :messages, Message
    belongs_to :user, User

    timestamps
end

schema "users" do
    field :name, :string
    has_many :chatGroups, ChatGroup

    timestamps
end

Any suggestions how to handle this?

Answer 1

This is an old question and the previously accepted answer was no longer the de facto way.

Ecto now supports HABTM or many to many associations.

https://hexdocs.pm/ecto/Ecto.Schema.html#many_to_many/3

many_to_many :users, MyApp.User, join_through: "chat_group_users"

Answer 2

Ecto has support for has_many/3 through relationships. This involves creating an intermediate table between your chat groups and your users.

You can do this with the following schema:

chat_group.ex:

schema "chat_groups" do
  has_many :chat_group_users, MyApp.ChatGroupUser
  has_many :users, through: [:chat_group_users, :user]
end

chat_group_user.ex:

schema "chat_group_users" do
  belongs_to :chat_group, MyApp.ChatGroup
  belongs_to :user, MyApp.User
end

You can also do the association the other way:

user.ex:

schema "users" do
  has_many :chat_group_users, MyApp.ChatGroupUsers
  has_many :chats, through: [:chat_group_users, :chat]
end

This allows you to do things like:

Repo.get(Chat, 1) |> Repo.preload(:users)

This will fetch the users for your chat model and populate the :user key with the value.