Adding IsUnique reusable domain rule

src/ORM/DomainChecker.php

@@ -15,6 +15,7 @@
 namespace Cake\ORM;
 
 use Cake\Datasource\EntityInterface;
+use Cake\ORM\Rule\IsUnique;
 
 /**
  * Contains logic for storing and checking domain rules on entities
@@ -65,4 +66,8 @@ public function checkUpdate(EntityInterface $entity) {
 		return $success;
 	}
 
+	public function isUnique(array $fields) {
+		return new IsUnique($fields);
+	}
+
 }

src/ORM/Rule/IsUnique.php

@@ -0,0 +1,39 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.0.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\ORM\Rule;
+
+use Cake\Datasource\EntityInterface;
+
+/**
+ *
+ *
+ */
+class IsUnique {
+
+	public function __construct(array $fields) {
+		$this->_fields = $fields;
+	}
+
+	public function __invoke(EntityInterface $entity, array $options) {
+		$conditions = $entity->extract($this->_fields);
+		if ($entity->isNew() === false) {
+			$keys = (array)$options['repository']->primaryKey();
+			$conditions['NOT'] = $entity->extract($keys);
+		}
+
+		return !$options['repository']->exists($conditions);
+	}
+
+}

tests/TestCase/ORM/DomainRulesIntegrationTest.php

@@ -278,4 +278,25 @@ public function testSaveBelongsToManyWithValidationErrorInJointEntityNonAtomic()
 		$this->assertEquals(5, $entity->tags[1]->_joinData->tag_id);
 	}
 
+/**
+ * Tests the isUnique domain rule
+ *
+ * @group save
+ * @return void
+ */
+	public function testIsUniqueDomainRule() {
+		$entity = new Entity([
+			'name' => 'larry'
+		]);
+
+		$table = TableRegistry::get('Authors');
+		$rules = $table->domainRules();
+		$rules->add($rules->isUnique(['name']));
+
+		$this->assertFalse($table->save($entity));
+
+		$entity->name = 'jose';
+		$this->assertSame($entity, $table->save($entity));
+	}
+
 }