'preload' results in 'Complex :in queries is not supported' error

[Qqwy]

When using an Ecto.Query.preload(:relation_name) in a pipeline with a query, the following error is thrown:

Complex :in queries is not supported by the Mnesia adapter.

This error is not indicative of what is happening here, so:

a) we need a better error.
b) maybe it is possible to support this feature after all?

[AndrewDryga]

@Qqwy can you contribute a test that reproduces it? We do support some :in queries, but they are emulated by nested :or's and don't work in all possible cases.

[Qqwy]

Of course! I am building a chat application, and this is what I am doing:

Schemas:

defmodule MyApp.Chat.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    belongs_to :app, MyApp.Chat.App
    field :name, :string
    has_many :sent_messages, MyApp.Chat.Message

    timestamps()
  end
end

defmodule MyApp.Chat.Message do
  use Ecto.Schema
  import Ecto.Changeset

  schema "message" do
    belongs_to :sender, MyApp.Chat.User
    belongs_to :conversation, MyApp.Chat.Conversation
    field :content, :string

    timestamps()
  end
end

Migrations:

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

  def change do
    create table(:users) do
      add :name, :string
      add :app_id, :integer #references("apps")

      timestamps()
    end
  end
end

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

  def change do
    create table(:message) do
      add :sender_id, :integer # references("users")
      add :conversation_id, :integer # references("conversations")
      add :content, :string

      timestamps()
    end
  end
end

(For this example, the 'conversation' schema and 'apps' schema are not really important)

I then attempt the following query:

      from(MyApp.Chat.Message, where: [conversation_id: ^conversation_id], order_by: [desc: :inserted_at], limit: 10)
      |> preload(:sender)
      |> MyApp.Repo.all()

to fetch all Messages which should each contain there respective sender.
Executing this query fails with the error shown above (Complex :in queries is not supported by the Mnesia adapter.).

---

I am not entirely sure how to extract a test out of this, but hopefully it does give you some indication of what is going on. (I try to run Ecto.Query.preload on a field that is a belongs_to in my schema).