Rest and JPA, force Jersey the use of auto-increment instead of JSON object value?

Question

Ok so I am using Jersey for my REST services, and I'm trying to persist a batch of objects received in JSON.

This is my entity:

@Entity
@Table(name="element", uniqueConstraints=@UniqueConstraint(columnNames={"key","value"}))
public classElement implements Serializable {

    @Id
@NotNull
private long key;

@NotBlank
@Size(max=30)
private String value;

@NotBlank
@Size(max=12)
private String more;

@NotNull 
private long userid;

And this is my "create" method:

@POST
@Path("/create")
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON) 
public Response createRecord(List<Element> elements) throws Exception{

    for(Element element : elements){

        violations = validator.validate(element);       
        if (violations.size() > 0) {

         jsonResponse.setSuccess(false);
         jsonResponse.setMessage("Error");
         jsonResponse.setValidationError(this.getValidationErrorList());    

        } 
        else {
            em.persist(valeur);  
        }
        jsonResponse.setSuccess(true);
        jsonResponse.setMessage("Success");
 }


    return Response.ok().entity(new ObjectMapper().writeValueAsString(jsonResponse)).build();

}

This works nicely when I have a json containing the value, more, and userid fields. The key gets an auto-incremented value. But the problem is, in my context I can't validate everything the "user" sends. So it is possible to send the json with the above fields AND the key (primary key). So I get a SQLError (duplicate entry) when it is persisted.

So my question (finally): What exactly happens when I do not specify any key and it gets an "auto" value, and how can I reproduce this even when the key is specify? (I.E. ignore the json key field and have the same auto-incremented value I would get without the field)

Thanks a lot!