What is the connection between validate() and hasErrors()

Question

This question comes from the problem of another question of mine. In that question, I come across a situation that hasErrors() function doesn't work for non-persistent domain class, even after all the things I did following the instruction, part 7.5 .

Following Victor's way, I fixed the problem by calling validate(), but I don't understand why it works. The Grails documents seem to say nothing about you should call a validate() before hasErrors() function. How could this happen?

Answer 1

It does make sense to me that validate would need to be called before asking an object whether it hasErrors (or save for proper domain objects, which calls validate under the covers). Validate in this context means "check whether this object is valid and indicate any errors if not".

Alternatively the GORM implementation would have to call validate every time any change is made to an object, which to me would be less desirable behaviour, as it might involve lots of work being done often and unnecessarily (some of those constraints could involve a lot of work).

The beginning of section 7.2 states pretty clearly "To validate a domain class you can call the validate method on any instance". It also states that "within Grails there are essentially 2 phases of validation, the first phase is data binding which occurs when you bind request parameters onto an instance such as... At this point you may already have errors in the errors property due to type conversion (such as converting Strings to Dates). You can check these and obtain the original input value using the Errors API. ... The second phase of validation happens when you call validate or save. This is when Grails will validate the bound values againts the constraints you defined."

The documentation for hasErrors also mentions this. You can access this by finding the method call in the navigation frame on the left, when you are on the documentation site. I would always recommend looking on these as well as the more descriptive user guide pages, as they often give a little more detail. Here's the page for the validate method too.

I've never had a problem calling validate directly - it's very clear to me and I can choose the point where all the work is done and I'm ready for the validation to take place. I can't see an option to change this behaviour anywhere, but if you wanted validate to be called automatically or under certain conditions, you could perhaps use some Groovy meta programming magic by maybe adding invokeMethod to the class and have it call validate before passing certain calls on. Have a look here and here. (Not sure I would recommend that though! And bear in mind your class would now be dependent on being used within the GORM validation framework, as that validate method might not otherwise exist).