title: Angular UI Tree - Allow only drag & drop into second level (children nodes)
body: <p>I am currently using <a href="https://angular-ui-tree.github.io/angular-ui-tree/#/basic-example" rel="nofollow">Angular UI Tree</a>.</p>
  
  <p>My object:</p>
  
  <pre><code>     [
            {
              "id": 1,
              "title": "class1",
              "students": [
                {
                  "id": 11,
                  "title": "student1.1",
                },
                {
                  "id": 12,
                  "title": "student1.2"
                }
              ]
            },
            {
              "id": 2,
              "title": "class2",
              "students": []
            },
            {
              "id": 3,
              "title": "class3",
              "students": [
                {
                  "id": 31,
                  "title": "student3.1"
                }
              ]
            }
          ]
  </code></pre>
  
  <p>What I want to achieve is to allow students to drag &amp; drop inside classes (The classes do not have to be draggable, and students do not have to be dropped into first level (classes) only to a second level (students)).</p>
  
  <p>Is this possible to achieve with Angular UI Tree?</p>