Exclude through associations from __schema__(:associations)

[Adzz]

Precheck

Environment

• Elixir version (elixir -v): Elixir 1.10.2 (compiled with Erlang/OTP 22)
• Ecto version (mix deps): 3.4.4
• Database adapter and version (mix deps):

Current behavior

Currently if you try to cast a through association you get the following (somewhat misleading) error message:

defmodule Like do
  use Ecto.Schema
  schema "likes" do
    field(:up, :boolean)
  end
end

defmodule Comment do
  use Ecto.Schema
  schema "comments" do
    has_many(:likes, Like)
  end
end

defmodule Post do
 use Ecto.Schema
  schema "posts" do
    has_many(:comments, Comment)
    has_many(:comment_likes, through: [:comments, :like])
  end
end

Ecto.Changeset.cast(%Post{}, %{}, Post.__schema__(:fields)) 
|> Ecto.Changeset.cast_assoc(:comment_likes)

** (ArgumentError) cannot cast assoc `comment_likes`, assoc `comment_likes` not found. Make sure it is spelled correctly and properly pluralized (or singularized)

I think I can see why you can't cast_assoc a through association (though I think we could improve the error message!), but this trips me up when I try to automatically cast all associations with reflection.

Essentially if I do this: Post.__schema__(:associations) to find all of the associations that I need to cast, comment_likes is in the list.

The only way I can think to get this information right now is to use __changeset__ to figure out the list of associations, but that takes a lot more work.

Expected behavior

I'm not sure whether we could:

1. Add a reflection mechanism for through associations, so I can filter them out.
   or
2. Exclude through associations from __schema__(:associations)
   and / or
3. improve the error message.

[josevalim]

Use __schema__(:associations) and then check if the association is in __changeset__. That should fix your case. I will work on improving the error message. Thanks!

[Adzz]

Thanks José!