Use concrete entities on missing data.

When an association is undefined return a concrete entity class. This is
necessary as notEmpty/allowEmpty callbacks could reference the entity
provider.

Refs #5190

src/View/Form/EntityContext.php

@@ -268,7 +268,8 @@ protected function _getEntity($path) {
 			$isLast = ($i === $last);
 
 			if (!$isLast && $next === null && $prop !== '_ids') {
-				return new Entity();
+				$table = $this->_getTable($path);
+				return $table->newEntity();
 			}
 
 			$isTraversable = (

tests/TestCase/View/Form/EntityContextTest.php

@@ -29,6 +29,15 @@
  * Test stub.
  */
 class Article extends Entity {
+
+/**
+ * Testing stub method.
+ *
+ * @return bool
+ */
+	public function isRequired() {
+		return true;
+	}
 }
 
 /**
@@ -647,6 +656,37 @@ public function testIsRequiredAssociatedHasMany() {
 		$this->assertFalse($context->isRequired(''));
 	}
 
+/**
+ * Test isRequired on associated entities with custom validators.
+ *
+ * Ensures that missing associations use the correct entity class
+ * so provider methods work correctly.
+ *
+ * @return void
+ */
+	public function testIsRequiredAssociatedCustomValidator() {
+		$this->_setupTables();
+		$users = TableRegistry::get('Users');
+		$articles = TableRegistry::get('Articles');
+
+		$validator = $articles->validator();
+		$validator->notEmpty('title', 'nope', function ($context) {
+			return $context['providers']['entity']->isRequired();
+		});
+		$articles->validator('default', $validator);
+
+		$row = new Entity([
+			'username' => 'mark'
+		]);
+		$context = new EntityContext($this->request, [
+			'entity' => $row,
+			'table' => 'Users',
+			'validator' => 'default',
+		]);
+
+		$this->assertTrue($context->isRequired('articles.0.title'));
+	}
+
 /**
  * Test isRequired on associated entities.
  *
@@ -943,6 +983,7 @@ protected function _setupTables() {
 		$articles = TableRegistry::get('Articles');
 		$articles->belongsTo('Users');
 		$articles->hasMany('Comments');
+		$articles->entityClass(__NAMESPACE__ . '\Article');
 
 		$comments = TableRegistry::get('Comments');
 		$users = TableRegistry::get('Users');