Allow belongs_to via changesets

[josevalim]

The main blocker for this feature in the past was how callbacks would work. Given callbacks are on the way out, this feature will be straight-forward to implement. It is pending on callbacks being effectively removed.

[josevalim]

We should also support on_delete for belongs_to once we add this.

[josevalim]

This is now in master.

@paulcsmith, could you please check if, when using Ecto master, ExMachina no longer needs any of the Ecto code? For example, we support inserting the whole tree directly, like here:

https://github.com/elixir-lang/ecto/blob/6f1971f4120b84e1a441792feb77ba451c4fc783/integration_test/cases/repo.exs#L720-L743

[paulcsmith]

@josevalim This will be so awesome! I was able to remove all of the Ecto code and still have almost all the tests passing

It almost worked. These two tests failed when the association was %Ecto.Association.NotLoaded{}

https://github.com/thoughtbot/ex_machina/blob/4443b72b677387a09fbc6c1351b29c526a0ca061/test/ex_machina/ecto_has_many_test.exs#L120

https://github.com/thoughtbot/ex_machina/blob/4443b72b677387a09fbc6c1351b29c526a0ca061/test/ex_machina/ecto_test.exs#L142

Here's the stack trace:

** (FunctionClauseError) no function clause matching in Ecto.Changeset.Relation.do_empty/1
     stacktrace:
       (ecto) lib/ecto/changeset/relation.ex:24: Ecto.Changeset.Relation.do_empty(nil)
       (ecto) lib/ecto/repo/schema.ex:377: anonymous fn/4 in Ecto.Repo.Schema.surface_changes/4
       (elixir) lib/enum.ex:1473: Enum."-reduce/3-lists^foldl/2-0-"/3
       (ecto) lib/ecto/repo/schema.ex:366: Ecto.Repo.Schema.surface_changes/4
       (ecto) lib/ecto/repo/schema.ex:128: Ecto.Repo.Schema.do_insert/4
       (ecto) lib/ecto/repo/schema.ex:76: Ecto.Repo.Schema.insert!/4
       test/ex_machina/ecto_test.exs:141

I'll pull the latest Ecto and see if it's a problem in Ecto or in ExMachina and Ecto together. I'll open a PR if there's a problem in Ecto

[paulcsmith]

@josevalim So Ecto is totally fine. It fails if you explicitly set an Ecto.Association.NotLoaded without a cardinality, which would never happen in real life. I just did it in the test to make sure it handled unloaded associations.

So I just need to update the tests in ExMachina once Ecto 2.0 drops. Ecto side looks great 👍

[josevalim]

That's so awesome, I am so excited! :D