Fix fatal errors when cascaded deletes are used with MySQL.

We cannot use unbuffered queries to delete has many associations as the
nested delete cannot be excuted while the previous cursor is still open.

Refs #5637

src/ORM/Association/DependentDeleteTrait.php

@@ -44,7 +44,7 @@ trait DependentDeleteTrait
         $conditions = array_combine($foreignKey, $entity->extract($primaryKey));
 
         if ($this->_cascadeCallbacks) {
-            $query = $this->find('all')->where($conditions)->bufferResults(false);
+            $query = $this->find('all')->where($conditions);
             foreach ($query as $related) {
                 $table->delete($related, $options);
             }

tests/TestCase/ORM/Association/HasManyTest.php

@@ -480,10 +480,8 @@ class HasManyTest extends TestCase
         $query->expects($this->any())
             ->method('getIterator')
             ->will($this->returnValue($iterator));
-        $query->expects($this->once())
-            ->method('bufferResults')
-            ->with(false)
-            ->will($this->returnSelf());
+        $query->expects($this->never())
+            ->method('bufferResults');
 
         $this->article->expects($this->once())
             ->method('find')

tests/TestCase/ORM/TableTest.php

@@ -1965,8 +1965,7 @@ class TableTest extends TestCase
             'dependent' => true,
         ]);
 
-        $query = $table->find('all')->where(['id' => 1]);
-        $entity = $query->first();
+        $entity = $table->get(1);
         $result = $table->delete($entity);
 
         $articles = $table->association('articles')->target();
@@ -1979,6 +1978,25 @@ class TableTest extends TestCase
     }
 
     /**
+     * Test delete with dependent records
+     *
+     * @return void
+     */
+    public function testDeleteDependentHasMany()
+    {
+        $table = TableRegistry::get('authors');
+        $table->hasMany('articles', [
+            'foreignKey' => 'author_id',
+            'dependent' => true,
+            'cascadeCallbacks' => true,
+        ]);
+
+        $entity = $table->get(1);
+        $result = $table->delete($entity);
+        $this->assertTrue($result);
+    }
+
+    /**
      * Test delete with dependent = false does not cascade.
      *
      * @return void