Model validation placeholders. Fixes #772

system/Model.php

@@ -1150,7 +1150,11 @@ public function validate($data): bool
 		}
 		else
 		{
-			$this->validation->setRules($this->validationRules, $this->validationMessages);
+			// Replace any placeholders (i.e. {id}) in the rules with
+			// the value found in $data, if exists.
+			$rules = $this->fillPlaceholders($this->validationRules, $data);
+
+			$this->validation->setRules($rules, $this->validationMessages);
 			$valid = $this->validation->run($data);
 		}
 
@@ -1159,6 +1163,48 @@ public function validate($data): bool
 
 	//--------------------------------------------------------------------
 
+	/**
+	 * Replace any placeholders within the rules with the values that
+	 * match the 'key' of any properties being set. For example, if
+	 * we had the following $data array:
+	 *
+	 * [ 'id' => 13 ]
+	 *
+	 * and the following rule:
+	 *
+	 *  'required|is_unique[users,email,id,{id}]'
+	 *
+	 * The value of {id} would be replaced with the actual id in the form data:
+	 *
+	 *  'required|is_unique[users,email,id,13]'
+	 *
+	 * @param array $rules
+	 * @param array $data
+	 *
+	 * @return array
+	 */
+	protected function fillPlaceholders(array $rules, array $data)
+	{
+		$replacements = [];
+
+		foreach ($data as $key => $value)
+		{
+			$replacements["{{$key}}"] = $value;
+		}
+
+		if (! empty($replacements))
+		{
+			foreach ($rules as &$rule)
+			{
+				$rule = strtr($rule, $replacements);
+			}
+		}
+
+		return $rules;
+	}
+
+	//--------------------------------------------------------------------
+
 	/**
 	 * Returns the model's defined validation rules so that they
 	 * can be used elsewhere, if needed.

tests/_support/Models/ValidModel.php

@@ -15,13 +15,14 @@ class ValidModel extends Model
     protected $allowedFields = ['name', 'description'];
 
     protected $validationRules = [
-        'name' => 'required|min_length[3]'
+        'name'  => 'required|min_length[3]',
+	    'token' => 'in_list[{id}]'
     ];
 
     protected $validationMessages = [
         'name' => [
             'required' => 'You forgot to name the baby.',
-            'min_length' => 'Too short, man!'
+            'min_length' => 'Too short, man!',
         ]
     ];
 }

tests/system/Database/Live/ModelTest.php

@@ -424,7 +424,33 @@ public function testValidationBasics()
 
     //--------------------------------------------------------------------
 
-    public function testSkipValidation()
+	public function testValidationPlaceholdersSuccess()
+	{
+		$model = new ValidModel($this->db);
+
+		$data = [
+			'name' => 'abc',
+			'id' => 13,
+			'token' => 13
+		];
+
+		$this->assertTrue($model->validate($data));
+	}
+
+	public function testValidationPlaceholdersFail()
+	{
+		$model = new ValidModel($this->db);
+
+		$data = [
+			'name' => 'abc',
+			'id' => 13,
+			'token' => 12
+		];
+
+		$this->assertFalse($model->validate($data));
+	}
+
+	public function testSkipValidation()
     {
         $model = new ValidModel($this->db);
 

user_guide_src/source/database/model.rst

@@ -456,6 +456,37 @@ and simply set ``$validationRules`` to the name of the validation rule group you
 		protected $validationRules = 'users';
 	}
 
+Validation Placeholders
+-----------------------
+
+The model provides a simple method to replace parts of your rules based on data that's being passed into it. This
+sounds fairly obscure but can be especially handy with the ``is_unique`` validation rule. Placeholders are simply
+the name of the field (or array key) that was passed in as $data surrounded by curly brackets. It will be
+replaced by the **value** of the matched incoming field. An example should clarify this::
+
+    protected $validationRules = [
+        'email' => 'required|valid_email|is_unique[users.email,id,{id}]'
+    ];
+
+In this set of rules, it states that the email address should be unique in the database, except for the row
+that has an id matching the placeholder's value. Assuming that the form POST data had the following::
+
+    $_POST = [
+        'id' => 4,
+        'email' => '[email protected]'
+    ]
+
+then the ``{id}`` placeholder would be replaced with the number **4**, giving this revised rule::
+
+    protected $validationRules = [
+        'email' => 'required|valid_email|is_unique[users.email,id,4]'
+    ];
+
+So it will ignore the row in the database that has ``id=4`` when it verifies the email is unique.
+
+This can also be used to create more dynamic rules at runtime, as long as you take care that any dynamic
+keys passed in don't conflict with your form data.
+
 Protecting Fields
 -----------------