Internal server error when validation fails on a join model

Hello!

I have a model with a has_many :through relationship and a couple of validations on the join model. Whenever the validations fail i get a 500 response, a 422 with a properly formatted jsonapi error response would be preferable. For now i've resorted to using custom exceptions and fail CustomException in the validations so it's catched properly, but that's not ideal.

Problem is, the shovel (<<) method in _create_to_many_links (https://github.com/cerebris/jsonapi-resources/blob/master/lib/jsonapi/resource.rb#L280) is triggering the model's save! immediately, so when it raises ActiveRecord::ValidationFailed it's not catched and interpreted as an internal server error.

Can this be solved, or is there a preferred approach to processing such exceptions?

I've added a failing spec that demonstrates this issue here: https://github.com/needto/jsonapi-resources/commit/928485a4ea071edf164e954496c06b0de314d555

Start in lib/jsonapi/resource.rb at _create_to_many_links around line 280, where the issue reports that the join model is saved immediately. Run or inspect the failing spec from commit 928485a4ea071edf164e954496c06b0de314d555; done means validation failures produce a 422 response with a properly formatted JSON:API error response instead of a 500.