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

Comienza en lib/jsonapi/resource.rb, en _create_to_many_links, alrededor de la línea 280, donde el issue informa que el modelo de unión se guarda inmediatamente. Ejecuta o inspecciona la spec que falla del commit 928485a4ea071edf164e954496c06b0de314d555; se considera terminado cuando los errores de validación producen una respuesta 422 con una respuesta de error JSON:API correctamente formateada en lugar de una 500.