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Data validation is an important part of any application, as it helps to make sure that the data in a Model conforms to the business rules of the application. For example, you might want to make sure that passwords are at least eight characters long, or ensure that usernames are unique. Defining validation rules makes form handling much, much easier.

There are many different aspects to the validation process. What we’ll cover in this section is the model side of things. Essentially: what happens when you call the save() method of your model. For more information about how to handle the displaying of validation errors, check out the section covering FormHelper.

The first step to data validation is creating the validation rules in the Model. To do that, use the Model::validate array in the Model definition, for example:

<?php
class User extends AppModel {  
    var $name = 'User';
    var $validate = array();
}
?>

1. <?php
2. class User extends AppModel {
3. var $name = 'User';
4. var $validate = array();
5. }
6. ?>

In the example above, the $validate array is added to the User Model, but the array contains no validation rules. Assuming that the users table has login, password, email and born fields, the example below shows some simple validation rules that apply to those fields:

<?php
class User extends AppModel {
    var $name = 'User';
    var $validate = array(
        'login' => 'alphaNumeric',
        'email' => 'email',
        'born' => 'date'
    );
}
?>

1. <?php
2. class User extends AppModel {
3. var $name = 'User';
4. var $validate = array(
5. 'login' => 'alphaNumeric',
6. 'email' => 'email',
7. 'born' => 'date'
8. );
9. }
10. ?>

This last example shows how validation rules can be added to model fields. For the login field, only letters and numbers will be accepted, the email should be valid, and born should be a valid date. Defining validation rules enables CakePHP’s automagic showing of error messages in forms if the data submitted does not follow the defined rules.

CakePHP has many validation rules and using them can be quite easy. Some of the built-in rules allow you to verify the formatting of emails, URLs, and credit card numbers – but we’ll cover these in detail later on.

Here is a more complex validation example that takes advantage of some of these built-in validation rules:

<?php
class User extends AppModel {
    var $name = 'User';
    var $validate = array(
        'login' => array(
            'alphaNumeric' => array(
                'rule' => 'alphaNumeric',
                'required' => true,
                'message' => 'Alphabets and numbers only'
                ),
            'between' => array(
                'rule' => array('between', 5, 15),
                'message' => 'Between 5 to 15 characters'
            )
        ),
        'password' => array(
            'rule' => array('minLength', '8'),
            'message' => 'Mimimum 8 characters long'
        ),
        'email' => 'email',
        'born' => array(
            'rule' => 'date',
            'message' => 'Enter a valid date',
            'allowEmpty' => true
        )
    );
}
?>

1. <?php
2. class User extends AppModel {
3. var $name = 'User';
4. var $validate = array(
5. 'login' => array(
6. 'alphaNumeric' => array(
7. 'rule' => 'alphaNumeric',
8. 'required' => true,
9. 'message' => 'Alphabets and numbers only'
10. ),
11. 'between' => array(
12. 'rule' => array('between', 5, 15),
13. 'message' => 'Between 5 to 15 characters'
14. )
15. ),
16. 'password' => array(
17. 'rule' => array('minLength', '8'),
18. 'message' => 'Mimimum 8 characters long'
19. ),
20. 'email' => 'email',
21. 'born' => array(
22. 'rule' => 'date',
23. 'message' => 'Enter a valid date',
24. 'allowEmpty' => true
25. )
26. );
27. }
28. ?>

Two validation rules are defined for login: it should contain letters and numbers only, and its length should be between 5 and 15. The password field should be a minimum of 8 characters long. The email should be a valid email address, and born should be a valid date. Also, notice how you can add custom messages that CakePHP will show when these validation rules fail.

As the example above shows, a single field can have multiple validation rules. And if the built-in rules do not match your criteria, you can always add your own validation rules as required.

Now that you’ve seen the big picture on how validation works, let’s look at how these rules are defined in the model. There are three different ways that you can define validation rules: simple arrays, single rule per field, and multiple rules per field.