Many to many self referencing schema for user following

Phoenix Forum Questions / Help
,
1

Hi guys,

I’m trying to build something similar to Twitter where users can follow other users. I’m not sure exactly how to properly create self-referencing schema in this case.

Migrations:

defmodule MyApp.Repo.Migrations.CreateUsers do
  use Ecto.Migration

  def change do
    create table(:users) do
      add :username, :string
    end
  end
end

defmodule MyApp.Repo.Migrations.CreateFollows do
  use Ecto.Migration

  def change do
    create table(:followings) do
      add :follower_id, references(:users, on_delete: :delete_all)
      add :followed_id, references(:users, on_delete: :delete_all)
      timestamps()
    end

    create index(:followings, [:follower_id, :followed_id], unique: true)
  end
end

Schemas:

defmodule MyApp.User do
  use Ecto.Schema

  schema "users" do
    field :username, :string

    ...
  end
end

defmodule MyApp.Follow do
  use Ecto.Schema

  schema "followings" do
    ---
  end
end

Can someone help me write the schemas properly using many_to_many please?

2

Did you try following this example in the docs? Basically it’s just a normal schema that you pass to the join_through option of many_to_many instead of the table name directly. Although it’s maybe worth noting that you don’t need to use a join schema. Personally I often prefer to avoid dealing with non-intuitive concept names like a “Follow”…

3

I did check those pages abut I’m not entirely sure which approach even to take as it’s the self-referencing schema.

4

What part of the self-referencing is causing you problems?

5

To be honest, I don’t even know… I also don’t like those abstract names, what do you think about a table called “followings” with “user_id” and a “follower_id”?

schema "users" do
    field :name, :string

    many_to_many :users, User, join_through: Followings, join_keys: [user_id: :id, follower_id: :id]
    many_to_many :followers, User, join_through: Followings, join_keys: [follower_id: :id, user_id: :id]
  end

schema "followings" do
    field :user_id, :id
    field :follower_id, :id
  end