Add test case to prevent future regression.

Refs #8841

tests/TestCase/ORM/RulesCheckerIntegrationTest.php

@@ -15,6 +15,7 @@
 namespace Cake\Test\TestCase\ORM;
 
 use Cake\ORM\Entity;
+use Cake\ORM\RulesChecker;
 use Cake\ORM\TableRegistry;
 use Cake\TestSuite\TestCase;
 
@@ -29,7 +30,7 @@ class RulesCheckerIntegrationTest extends TestCase
      *
      * @var array
      */
-    public $fixtures = ['core.articles', 'core.articles_tags', 'core.authors', 'core.tags'];
+    public $fixtures = ['core.articles', 'core.articles_tags', 'core.authors', 'core.tags', 'core.categories'];
 
     /**
      * Tear down
@@ -461,6 +462,30 @@ class RulesCheckerIntegrationTest extends TestCase
     }
 
     /**
+     * Test ExistsIn on a new entity that doesn't have the field populated.
+     *
+     * This use case is important for saving records and their
+     * associated belongsTo records in one pass.
+     *
+     * @return void
+     */
+    public function testExistsInNotNullValueNewEntity()
+    {
+        $entity = new Entity([
+            'name' => 'A Category',
+        ]);
+        $table = TableRegistry::get('Categories');
+        $table->belongsTo('Categories', [
+            'foreignKey' => 'parent_id',
+            'bindingKey' => 'id',
+        ]);
+        $rules = $table->rulesChecker();
+        $rules->add($rules->existsIn('parent_id', 'Categories'));
+        $this->assertTrue($table->checkRules($entity, RulesChecker::CREATE));
+        $this->assertEmpty($entity->errors('parent_id'));
+    }
+
+    /**
      * Tests exists in uses the bindingKey of the association
      *
      * @return