`validates_uniqueness_of`¶

The validates_uniqueness_of validation in ActiveRecord allows you to validate whether an attribute's value is unique among the records in the database table. It ensures that the attribute's value is not duplicated within the dataset. To use validates_uniqueness_of, you need to specify the following options:

Attribute Name: This option specifies the name of the attribute you want to validate for uniqueness. It is typically set as a symbol or string.
```
static $validates_uniqueness_of = ['email' => []];
```

Message (Optional): To customize the error message displayed when the validation fails, you can use the message option to provide a custom error message.

static $validates_uniqueness_of = [
    'username' => [
        'message' => 'This username is already in use. Please choose a different one.',
    ],
];

Scope (Optional): The scope option allows you to specify additional conditions for uniqueness validation. It is used when you want to ensure uniqueness based on certain criteria. You can specify an array of attribute names to scope the uniqueness check.
```
static $validates_uniqueness_of = [
    'email' => [
        'scope' => ['account_type', 'status'],
        'message' => 'Email address must be unique for the selected account type and status.',
    ],
];
```
Allow Blank (Optional): The allow_blank option allows you to specify whether the attribute is allowed to be empty (e.g., an empty string). By default, allow_blank is set to false, meaning that the attribute must not be blank. You can set it to true to allow blank values.
```
static $validates_uniqueness_of = [
    'email' => [
        'allow_blank' => true
    ]
];
```
Allow Null (Optional): The allow_null option allows you to specify whether the attribute is allowed to be null. By default, allow_null is set to false, meaning that the attribute must not be null. You can set it to true to allow null values.
```
static $validates_uniqueness_of = [
    'username' => [
        'allow_null' => true
    ]
];
```
On (Optional): The on option allows you to specify when the validation should occur. By default, it is set to 'save', which means the validation is performed when the record is saved. You can set it to 'create' to validate only during the creation of a new record or 'update' to validate only during updates.
```
static $validates_uniqueness_of = [
    'username' => [
        'on' => 'create'
    ]
];
```

Example of Usage:

class User extends ActiveRecord\Model {
    static $validates_uniqueness_of = ['email', 'username'];
}

validates_uniqueness_of¶

`validates_uniqueness_of`¶