D3: use nest function to turn flat data with parent key into a hierarchy

Question

I'm sure there's a really simple elegant way to do this but I can't quite figure it out. I have some input data that looks like this:

[
{id: 1, name: "Peter"},
{id: 2, name: "Paul", manager: 1},
{id: 3, name: "Mary", manager: 1},
{id: 4, name: "John", manager: 2},
{id: 5, name: "Jane", manager: 2}
]

If possible, I would like to use the d3.js nest operator to get a structure to use in the hierarchy layout. Like this:

[ 
   {name: "Peter", children: [
          {name:"Paul", children: [
              {name:"John"},
              {name:"Jane"}
          ]},
          {name:"Mary"}
      ]
   }
]

Answer 1

You can't use the nest operator here because nesting produces a fixed hierarchy: the number of levels in the output hierarchy is the same as the number of key functions you specify.

That said, you can write your own function which produces a tree. Assuming that the root node is the first node in the input array, you can create a map from id to node, and then construct the tree lazily.

function tree(nodes) {
  var nodeById = {};

  // Index the nodes by id, in case they come out of order.
  nodes.forEach(function(d) {
    nodeById[d.id] = d;
  });

  // Lazily compute children.
  nodes.forEach(function(d) {
    if ("manager" in d) {
      var manager = nodeById[d.manager];
      if (manager.children) manager.children.push(d);
      else manager.children = [d];
    }
  });

  return nodes[0];
}

If you know that the nodes are listed in order such that managers appear before their reports, you can simplify the code to iterate only once.