Schema validation errors on polymorphic embeddings are too vague and do not align with Ecto's `embeds_*`

[bvobart]

Hi @mathieuprog, how are you? :)

As mentioned in my Elixir forum thread here, I notice that the schema validation errors on polymorphic embeddings are very non-descriptive.

E.g. if I have a schema with a polymorphic embed on :children and data where one of those children has an invalid type field, then casting it with changeset results in an Ecto.Changeset struct with changeset.errors == [children: {"is invalid", []}]. Note that this does not contain any info about the particular element within :children that caused the error, or what the underlying error actually was.

I’ve created a minimal working example in a GitHub repo here just to illustrate the point a bit better: https://github.com/bvobart/polymorphic-embed-errors-mwe

See lib/schema.ex and test/schema_test.exs to see what I mean. I’ve added some tests that show the current behaviour and the behaviour that would be desirable (and that would be more akin to what Ecto does for embeds_many).

I found that this line is where the indescriptive error is added. I feel like it should be possible to add a more descriptive error in the Enum.map before it, i.e. here. Although I'm not sure what would be necessary to make it behave more like Ecto's embed_one and embed_many as shown in my example project's test case. It might be dependent on #74 but I'm not sure.

What would be the best solution for this?