Add support for value validation methods

Author: borkweb

README.md

@@ -183,6 +183,48 @@ class Breakfast_Model extends Model implements Contracts\ModelCrud {
 }
 ```
 
+## Attribute validation
+
+Sometimes it would be helpful to validate attributes that are set in the model. To do that, you can create `validate_*()`
+methods that will execute any time an attribute is set.
+
+Here's an example:
+
+```php
+namespace Boomshakalaka\Whatever;
+
+use Boomshakalaka\StellarWP\Models\Model;
+
+class Breakfast_Model extends Model {
+	/**
+	 * @inheritDoc
+	 */
+	protected $properties = [
+		'id'        => 'int',
+		'name'      => 'string',
+		'price'     => 'float',
+		'num_eggs'  => 'int',
+		'has_bacon' => 'bool',
+	];
+
+	/**
+	 * Validate the name.
+	 *
+	 * @param string $value
+	 *
+	 * @return bool
+	 */
+	public function validate_name( $value ): bool {
+		if ( ! preg_match( '/eggs/i', $value ) ) {
+			throw new \Exception( 'Breakfasts must have "eggs" in the name!' );
+		}
+
+		return true;
+	}
+}
+
+```
+
 ## Data Transfer Objects
 
 Data Transfer Objects (DTOs) are classes that help with the translation of database query results (or other sources of data)

src/Models/Model.php

@@ -204,6 +204,19 @@ protected function getRelationship( string $key ) {
 		return null;
 	}
 
+	/**
+	 * Returns true if an attribute exists. Otherwise, false.
+	 *
+	 * @since 1.1.0
+	 *
+	 * @param string $key Attribute name.
+	 *
+	 * @return bool
+	 */
+	protected function hasAttribute( string $key ) : bool {
+		return array_key_exists( $key, $this->attributes );
+	}
+
 	/**
 	 * Checks whether a relationship has already been loaded.
 	 *
@@ -327,6 +340,11 @@ public function setAttribute( string $key, $value ) : ModelInterface {
 		$this->validatePropertyExists( $key );
 		$this->validatePropertyType( $key, $value );
 
+		$validation_method = 'validate_' . $key;
+		if ( method_exists( $this, $validation_method ) ) {
+			$this->$validation_method( $value );
+		}
+
 		$this->attributes[ $key ] = $value;
 
 		return $this;