Using nodes with parents instead of nodes with children in Angular UI Tree

Question

I'm using Angular UI Tree to show a TreeView. To request all nodes to a REST API, I use restangular to return a list of nodes.

But, Angular UI Tree uses this data structure:

[
  { 
    "id" : 1,
    "nodes": [
        {
            "id" : 11
        }
    ]
  },
  { 
    "id" : 2,
    "nodes": [
        {
            "id" : 21
        }
    ]
  }
]

And I want it to use:

 [
   {"id" :1, parent : ""},
   {"id" :2, parent : ""},
   {"id" :21, parent : "2"},
   {"id" :11, parent : "1"}
 ] 

How can I do that on template?

I'm using this.

Is there any other way to request an array of nodes and to use that in UI Tree where each node is a Restangular object?