ActiveModel adapter 422 error handling

[fsmanuel]

@igorT Is it possible to consider the attrs hash here?

Right now the errors from the server doesn't match custom mappings and are worthless.

[igorT]

Yes, this seems like a bug. I'll try and make a PR, but it might need to be a part of a slightly bigger refactor.

[ahacking]

I have been looking at how errors are handled in Ember Data as I want to ensure the following:

• payload error keys get mapped to model keys consistently with how the serializer maps payload keys in the success case
• embedded models (ie using the embeded_records_mixin) also get their errors property setup

This is really a common problem for all serializers, not just ActiveModel as the store currently by-passes the serializer when there is an InvalidError and pushes errors directly into the model here.

A possible solution would be to introduce extractInvalid on the serializer class and call extract from the store here passing 'Invalid' as the operation type analogous to what is done in the success case a few lines above.

@igorT Has there been any progress on this?

The reason I ask is I need to solve this for a project pretty urgently so if there is some existing work I can base my changes on I will look at that, or if you can recommend a better way forward I will consider that too.

I will also look at what other enhancements are needed so that embedded record handling can extract errors pertaining to those record instances.

[igorT]

@ahacking I haven't worked on this personally, your PR looks great.