defmodule App.Newsletters.Newsletter do
  use App.Schema
  import Ecto.Changeset

  schema "newsletters" do
    field :frequency, :string
    field :name, :string
    field :sender_name, :string
    field :email, :string

    field :selected?, :boolean, virtual: true, default: false

    timestamps()
  end

  @doc false
  def changeset(newsletter, attrs) do
    newsletter
    |> cast(attrs, [:name, :frequency, :sender_name])
    |> validate_required([:name, :frequency, :sender_name])
  end
end




defmodule App.Digests.Digest do
  use App.Schema
  import Ecto.Changeset

  schema "digests" do
    field :frequency, :string
    field :name, :string
    belongs_to :organization, App.Organizations.Organization

    many_to_many(
      :newsletters,
      App.Newsletters.Newsletter,
      join_through: App.Digests.DigestsNewsletter,
      on_replace: :delete,
      join_keys: [digest_id: :id, newsletter_id: :id]
    )


    timestamps()
  end

  def for_id(query \\ __MODULE__, id) do
    query
    |> where([s], s.id == ^id)
  end

  @doc false
  def changeset(digest, attrs) do
    digest
    |> cast(attrs, [:name, :frequency])
    |> validate_required([:name, :frequency])
  end
end



defmodule App.Digests.DigestsNewsletter do
  @moduledoc """
  UserProject module
  """

  use App.Schema
  import Ecto.Changeset

  alias App.Digests.Digest
  alias App.Newsletters.Newsletter

  @already_exists "ALREADY_EXISTS"

  @primary_key false
  schema "newsletters_digests" do
    belongs_to(:digest, Digest, primary_key: true)
    belongs_to(:newsletter, Newsletter, primary_key: true)

    timestamps()
  end

  @required_fields ~w(newsletter_id user_id)a
  def changeset(newsletters_digests, params \\ %{}) do
    newsletters_digests
    |> cast(params, @required_fields)
    |> validate_required(@required_fields)
    |> foreign_key_constraint(:digest_id)
    |> foreign_key_constraint(:newsletter_id)
    |> unique_constraint([:digest, :newsletter],
      name: :digest_id_newsletter_id_unique_index,
      message: @already_exists
    )
  end
end



## Many to Many Migration
defmodule App.Repo.Migrations.CreateUserNewsletters do
  use Ecto.Migration

  def change do
    create table(:newsletters_digests, primary_key: false) do
      add(:digest_id, references(:digests, on_delete: :delete_all), primary_key: true)
      add(:newsletter_id, references(:newsletters, on_delete: :delete_all), primary_key: true)
      timestamps()
    end

    create(index(:newsletters_digests, [:digest_id]))
    create(index(:newsletters_digests, [:newsletter_id]))

    create(
      unique_index(:newsletters_digests, [:digest_id, :newsletter_id],
        name: :digest_id_newsletter_id_unique_index
      )
    )
  end
end

I have a many to many field that connects newsletters and digests. When I try to preload the many to many field newsletters via

    Repo.get!(Digest, id)
    |> Repo.preload(:newsletters)

I get the following error

[error] #PID<0.898.0> running AppWeb.Endpoint (connection #PID<0.880.0>, stream id 3) terminated
Server: localhost:4000 (http)
Request: GET /app/digest_live/mvMl2V
** (exit) an exception was raised:
    ** (Postgrex.Error) ERROR 42704 (undefined_object) type "serial" does not exist

    query: SELECT n0."id", n0."frequency", n0."name", n0."sender_name", n0."email", n0."inserted_at", n0."updated_at", n1."digest_id"::serial FROM "newsletters" AS n0 INNER JOIN "newsletters_digests" AS n1 ON n0."id" = n1."newsletter_id" WHERE (n1."digest_id" = ANY($1)) ORDER BY n1."digest_id"::serial
        (ecto_sql 3.8.0) lib/ecto/adapters/sql.ex:928: Ecto.Adapters.SQL.raise_sql_call_error/1
        (ecto_sql 3.8.0) lib/ecto/adapters/sql.ex:843: Ecto.Adapters.SQL.execute/6
        (ecto 3.8.1) lib/ecto/repo/queryable.ex:221: Ecto.Repo.Queryable.execute/4
        (ecto 3.8.1) lib/ecto/repo/queryable.ex:19: Ecto.Repo.Queryable.all/3
        (elixir 1.13.3) lib/enum.ex:1593: Enum."-map/2-lists^map/1-0-"/2
        (app 0.1.0) lib/app_web/live/digest_live.ex:23: AppWeb.DigestLive.apply_action/3
        (app 0.1.0) lib/app_web/live/digest_live.ex:19: AppWeb.DigestLive.handle_params/3
        (phoenix_live_view 0.17.6) lib/phoenix_live_view/utils.ex:369: anonymous fn/5 in Phoenix.LiveView.Utils.call_handle_params!/5
        (telemetry 1.1.0) /Users/shapathneupane/Developer/boilerplate_1.6/synopsis/deps/telemetry/src/telemetry.erl:320: :telemetry.span/3
        (phoenix_live_view 0.17.6) lib/phoenix_live_view/static.ex:271: Phoenix.LiveView.Static.call_mount_and_handle_params!/5
        (phoenix_live_view 0.17.6) lib/phoenix_live_view/static.ex:110: Phoenix.LiveView.Static.render/3
        (phoenix_live_view 0.17.6) lib/phoenix_live_view/controller.ex:39: Phoenix.LiveView.Controller.live_render/3
        (phoenix 1.6.6) lib/phoenix/router.ex:355: Phoenix.Router.__call__/2
        (app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.plug_builder_call/2
        (app 0.1.0) lib/plug/debugger.ex:136: AppWeb.Endpoint."call (overridable 3)"/2
        (app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.call/2

However, when I try to preload :organization in the digest schema. It works and preloads the organization.

   # this preloads organization without errors
    Repo.get!(Digest, id)
    |> Repo.preload(:organiaztion)



%App.Digests.Digest{
  __meta__: #Ecto.Schema.Metadata<:loaded, "digests">,
  frequency: "ONCE",
  id: "mvMl2V",
  inserted_at: ~U[2022-04-28 14:15:16Z],
  name: "ANOTHER DATASET",
  newsletters: #Ecto.Association.NotLoaded<association :newsletters is not loaded>,
  organization: %App.Organizations.Organization{
    __meta__: #Ecto.Schema.Metadata<:loaded, "organizations">,
    active: true,
    domain: nil,
    email: nil,
    id: 1,
    inserted_at: ~U[2022-04-28 03:01:21Z],
  },
  organization_id: "gKg12n",
  updated_at: ~U[2022-04-28 14:15:16Z]
}

Is there any changes to be made for many_to_many association preload?