hibernate unique key validation

Question

I have a field, say, user_name, that should be unique in a table.

What is the best way for validating it using Spring/Hibernate validation?

Answer 1

I've found kind of a tricky solution.

First, I've implemented the unique contraint to my MySql database :

CREATE TABLE XMLTAG
(
    ID INTEGER NOT NULL AUTO_INCREMENT,
    LABEL VARCHAR(64) NOT NULL,
    XPATH VARCHAR(128),
    PRIMARY KEY (ID),
    UNIQUE UQ_XMLTAG_LABEL(LABEL)
) ;

You see that I manage XML Tags that are defined by a unique label and a text field named "XPath".

Anyway, the second step is to simply catch the error raised when the user tries to do a bad update. A bad update is when trying to replace the current label by an existing label. If you leave the label untouched, no problemo. So, in my controller :

    @RequestMapping(value = "/updatetag", method = RequestMethod.POST)
    public String updateTag(
            @ModelAttribute("tag") Tag tag, 
            @Valid Tag validTag,
            BindingResult result,
            ModelMap map) {

        if(result.hasErrors()) {        // you don't care : validation of other
            return "editTag";       // constraints like @NotEmpty
        }
        else {
            try {
                tagService.updateTag(tag);    // try to update
                return "redirect:/tags";   // <- if it works        
            }
            catch (DataIntegrityViolationException ex) { // if it doesn't work
                result.rejectValue("label", "Unique.tag.label"); // pass an error message to the view 
                return "editTag"; // same treatment as other validation errors
            }
        }
    }

This may conflict with the @Unique pattern but you can use this dirty method to valid the adding too.

Note : there is still one problem : if other validation errors are catched before the exception, the message about unicity will not be displayed.