public function testIsLeaf()
 {
     $this->assertTrue(Tree::findOne(4)->isLeaf());
     $this->assertFalse(Tree::findOne(1)->isLeaf());
 }
 public function testLeaves()
 {
     $this->assertEquals(require __DIR__ . '/data/test-leaves-query.php', ArrayHelper::toArray(Tree::find()->leaves()->all()));
     $this->assertEquals(require __DIR__ . '/data/test-leaves-multiple-tree-query.php', ArrayHelper::toArray(MultipleTree::find()->leaves()->all()));
 }