Hello.

I’m using the excellent Absinthe package, I’m trying to create a many to many association with an Index table, it looks like this:

defmodule Vulkan.Course do
  use Vulkan.Web, :model

  schema "courses" do
    field :name, :string
    field :description, :string
    
    many_to_many :users, Vulkan.User, join_through: "user_courses"

    timestamps()
  end
  .
  .
  .
end

defmodule Vulkan.User do
  use Vulkan.Web, :model

  schema "users" do
    field :name, :string
    field :handle, :string
    field :email, :string
    field :is_admin, :boolean, default: false
    field :encrypted_password, :string
    
    field :password, :string, virtual: true
    
    many_to_many :courses, Vulkan.Course, join_through: "user_courses"

    timestamps()
  end
  .
  .
  .
end

And the index looks like this:

defmodule Vulkan.UserCourse do
  use Vulkan.Web, :model

  schema "user_courses" do
    belongs_to :user, Vulkan.User
    belongs_to :course, Vulkan.Course

    timestamps()
  end
end

Just in case, the migration:

defmodule Vulkan.Repo.Migrations.CreateUserCourse do
  use Ecto.Migration

  def change do
    create table(:user_courses) do
      add :user_id, references(:users, on_delete: :nothing), null: false
      add :course_id, references(:courses, on_delete: :nothing), null: false

      timestamps()
    end
    create index(:user_courses, [:user_id])
    create index(:user_courses, [:course_id])
    create index(:user_courses, [:user_id, :course_id], unique: true)
  end
end

Now my question is, how would I resolve the association? Should I create a type and resolve the association through it?

object :user_course do
    field :id, :id
    field :user_id, :id
    field :course_id, :id
end

object :user do
    field :id, :id
    field :name, :string
    field :handle, :string
    field :email, :string
    field :is_admin, :boolean
    field :posts, list_of(:post), resolve: assoc(:posts)
    field :courses, list_of(:course), resolve: assoc(:user_courses)
end

This approach doesn’t seem to work, because I’ve inserted a UserCourse with user_id; 1 and course_id: 1 (both exist in the database) and when I run the query

query posts {
  users {
    id
    name
    courses {
      id
      name
    }
  }
}

I get the response

{
  "data": {
    "users": [
      {
        "name": "Andrés Pérez",
        "id": "1",
        "courses": null
      },
      {
        "name": "Jon Doe",
        "id": "2",
        "courses": null
      }
    ]
  }
}

I can’t manage to find the docs for Absinthe-Ecto, so I’m quite lost.

How should I be doing it?

Any help is highly appreciated, thank you so much for your time.