Ecto: Ecto.Query.preload with module name is not working as expected

[b0oh]

Environment

• Elixir version: Elixir 1.4.2
• Database: PostgreSQL 9.6
• Ecto version (mix deps): 2.1.4
• Database adapter and version (mix deps): postgrex 0.13.2
• Operating system: macOS 10.12.6

Current behavior

Code sample:

defmodule App.Story do
  use App.Web, :model

  alias App.{Card, Placement}

  schema "stories" do
    has_many :placements, Placement
    has_many :cards, through: [:placements, :card]

    timestamps()
  end
end

defmodule App.Placement do
  use App.Web, :model

  alias App.{Card, Story}

  schema "placements" do
    belongs_to :card, Card
    belongs_to :story, Story

    timestamps()
  end
end

defmodule App.Card do
  use App.Web, :model

  alias App.Placement

  schema "cards" do
    has_many :placements, Placement

    timestamps()
  end
end

query = from(s in App.Story)
cards_query = from(c in App.Card) # works
cards_query = App.Card # doesn't work
query = preload(query, cards: ^cards_query)

App.Repo.all(query)

in second case it trows an error: schema App.Card does not have association App.Card

Expected behavior

Expected (according to @ericmj) both cases to behave the same.

[ericmj]

The preloader should call Ecto.Queryable.to_query/1 on all passed queryables instead of expecting them to already be %Ecto.Query{}.

[josevalim]

@ericmj the issue is that we also allow it to be an atom which is the association name and we can't call Ecto.Queryable on that (at least not for atoms). I think our best option here is to improve the error message if the atom starts with "Elixir.". Thoughts?

[ericmj]

Can we check if the atom is an association and then call the protocol?

[josevalim]

This means a typo in the association name will now become a much worse error message, no?

[ericmj]

We can still special handle the error message by checking Elixir. like you
suggested/

On Tue, Oct 17, 2017 at 8:17 PM, José Valim [e-mail]
wrote:

This means a typo in the association name will now become a much worse
error message, no?

—
You are receiving this because you were mentioned.
Reply to this email directly, view it on GitHub
https://github.com/elixir-ecto/ecto/issues/2279#issuecomment-337321127,
or mute the thread
https://github.com/notifications/unsubscribe-auth/AATV2hLDIf7BgYde4Fo3Z0YzRrrjU2Y9ks5stO9UgaJpZM4P8SdY
.

--
Eric Meadows-Jönsson

[josevalim]

Tackling this one as well!

[josevalim]

Fixed on master and backported to v2.2. For now just a better error message.