Disallowing the creation of tree cycles in the TreeBehavior

src/Model/Behavior/TreeBehavior.php

@@ -126,11 +126,18 @@ protected function _setParent($entity, $parent) {
 		$parentNode = $this->_getParent($parent);
 		$parentLeft = $parentNode->get($config['left']);
 		$parentRight = $parentNode->get($config['right']);
-
 		$right = $entity->get($config['right']);
 		$left = $entity->get($config['left']);
 
-		// Values for moving to the left
+		if ($parentLeft > $left && $parentLeft < $right) {
+			throw new \RuntimeException(sprintf(
+				'Cannot use node "%s" as parent for entity "%s"',
+				$parent,
+				$entity->get($this->_table->primaryKey())
+			));
+		}
+
+		// Values for moving )o the left
 		$diff = $right - $left + 1;
 		$targetLeft = $parentRight;
 		$targetRight = $diff + $parentRight - 1;

tests/TestCase/Model/Behavior/TreeBehaviorTest.php

@@ -486,4 +486,19 @@ public function testRootingSubTree() {
 		$this->assertEquals(range(1, 22), $numbers);
 	}
 
+/**
+ * Tests that trying to create a cycle throws an exception
+ *
+ * @expectedException RuntimeException
+ * @expectedExceptionMessage Cannot use node "5" as parent for entity "2"
+ * @return void
+ */
+	public function testReparentCycle() {
+		$table = TableRegistry::get('NumberTrees');
+		$table->addBehavior('Tree');
+		$entity = $table->get(2);
+		$entity->parent_id = 5;
+		$table->save($entity);
+	}
+
 }