Should validation be done in Form objects, or the model?

Question

This question is mainly geared towards Zend in PHP, although it certainly applies to other languages and frameworks, so I welcome everyone's opinion.

I've only recently been using the Zend framework, and while it's not perfect, I have had a pretty good time with it. One thing that drives me crazy, however, is that most of the examples I see of people using Zend do the validation in special form objects, rather than in the model. I think this is bad practice because data can enter into the system in other ways beyond form input, which means that either validators have to be bent and twisted to validate other input, or validation must be done in a second place, and logic duplicated.

I've found some other posts and blogs out there with people who feel the same way I do, but the developers of Zend made this choice for a reason, and other people seem to use it without issue, so I wanted to get some feedback from the community here.

As I said, this mainly applies to Zend, although I think it's important to look at the issue as a whole, rather than working within the confines of the Zend framework, since Zend was designed so that you could use as much, or as little, as you wished.

Answer 1

I am not aware of Zend. But.

Your model have to receive valid data. Model and it's methods shouldn't check data again and again. Of course there are should be functions that do actual validation and they should be called from the gui validation or from the other data input place.

The best you can do on your model side is call "Assertions" on all the data to be sure on the development time that validation have been taken its place.

The lower level of the code (UI, model, utils) the less validation and check code should be there. As then there is a big chance that the same validation will be called more then one.

Answer 2

Well, the validation can be done at many different levels and unsually none of them is "the best". Of course, the model can be populated with invalid data that do not come from the form, but we can also create forms whose data do not go to any model.

Moreover, the direct validation in models is unsually not integrated with our form rendering system, which causes problems if we want to show the error messages and re-populate the form with the user-entered data then.

Both of the solutions have their own pros and cons. It would be perfect to have a system that ensures us that the validation finally must be done at some level. If the form does not validate some data, the the model does and vice versa. Unfortunately, I haven't heard of such library, but I must note that the validators in the frameworks unsually are source-independent. You can pass the POST data to them, but the same can be done with the information retreived from a properly parsed CSV or other stuff.

Answer 3

It's important to remember that data validation which is relevant to an application isn't always the same thing as data validation that's relevant to a database schema.

Consider a simple registration form where a user creates an account with a username and password. You perform validation on the password because you want it to be X number of characters in length and contain a good mix of character types (or whatever).

But none of this is relevant to validate the data for database insertion, because you aren't going to store plain-text passwords - you're going to store a hash of them in some way (md5, md5 + salt, whatever). Instead you might make sure that you have a 32 character hexadecimal string so that it is very likely to be a properly created MD5 hash.

This password example isn't the only scenario, just a good one for explanation here in this topic.

So what's the answer? I don't think there's any one-solution-fits-all. Sometimes you will want (need?) to validate the data twice. Sometimes you'll do it once an only in the Model. Just match it as best as possible to your application's needs.

Answer 4

This is a non-zend specfic answer, however I believe that the model should be responsible for the validity of its own data. If this is the case then the validation belongs in the model, however this may not always be achievable and it may be necessary to perform validation in the view, however I think this should be in addition to the validation performed in the model not a replacement for it.

The problem with only having validation in the view is that at some point you will probably want another view on your data. Your site may become popular and customers are asking for XML based APIs to generate their own views. Do you then rely on the customer to validate the data?

Even if you do not have to provide APIs some customers may want customized views that are sufficiently different to warrant a completely different version of the page, again you now have validation in the views duplicated.

I think the ideal scenario is to have your model do the validation but to make the results of the validation available for the view to read and render the page again with the validation results displayed.

I think it is perfectly reasonable to have the view doing validation if you want to instantly display validation data back to the user etc but the final decision on data validity should rest with the model.

Answer 5

How about putting esthetical validation in the form, and business rules validation in the model.

Take a registration form for example.

The form would assure that the email field is trimmed and contains a valid email, that the password/confirm password field are identical and that the user checked the I Agree to terms checkbox.

The registration model would make sure that the email hasn't been taken yet in the table, would salt and hash the password.

It's how I split the two usually.

Answer 6

Perhaps you should have a look at Using Zend_Form in Your Models by Matthew Weier O'Phinney - one of the lead-developers of the Zend Framework - for his view on exactly this question.

Answer 7

User input should be validated when it is being inputted because it is specific to the form of entry (ie, do some form validation - make sure text boxes that should have numbers are numbers).

Business logic should probably be validated on the model because it is model specific (ie. make sure they have't already reserved that same room or something like that).

The problem with validating it at the model level is that the model might be used in different ways. Correct input for one scenario may not be correct input for another.

The other issue is that you usually want some context sensitive validation, such as displaying a red box around the form control that has the bad input.

The model or database might do some extra validation to make sure the user code isn't doing something completely wrong (constraints, etc).