How to handle Resource validation in REST webservices?

I'm building a REST webservice in Java using Spring, Jersey and Hibernate (JPA).

Now I'm trying to add support for validation in my resources. JSR-303 (Bean validation) naturally appeared as a suitable choice (I'm using Hibernate Validator which is the reference implementation of JSR-303). However, I'm trying to consolidate some concepts first.

It seems to me that there are at least two kinds of possible validations:

• Regular validation on fields, e.g. check that a property isn't null, check if a String property is a valid e-mail, check if an Integer property is greater than 10, etc. This is working as expected. I've registered a JAX-RS ExceptionMapper which maps javax.validation.ConstraintViolationException's into proper HTTP responses.
• Data Integrity Validation

I'll explain what I mean exactly by "Data Integrity Validation" with an example. This happens when there are relationships between two or more resources. Imagine two resources: a Product and a Category. A Product has a Category. When the client sends to the server the representation of the Product to create (a POST HTTP request), it must inform the Category of the product. The client must know the categories beforehand, of course. Imagine in JSON something like:

{
   "quantity": "10",
   "state": "PRODUCED",
   "category": {
      "id": "123"
   }
}

Well, the category with id 123 might not exist. If we try to insert this into the database, obviously we get a foreign key related exception. So I assume we have to validate these issues and return a proper message to the client, just as in regular property validations.

Now, my main questions are:

1. Right now I'm performing the validation in the Data Access