Finding adjacent nodes in a d3 force graph

Question

I'm aware this question has been asked several times, but it seems like the way I have my data set up is making this much more difficult than it should be. I'm trying to select nodes neighboring the node that the mouse hovers over so that I can lower the opacity of the non-neighboring nodes. I managed to do this to the links without any problem.

I've already tried re-writing Mike Bostock's solution here several times, but nothing works. At most, I can get every node to fade out when I hover over, which isn't helpful. This comment in a Google group makes me think that I need to have a section in my node data that tells what other nodes a node is connected to. My JSONs current look like this:

Nodes:

[
  {
    "id":0,
    "name":"Test Writer 1"
  },
  {
    "id":1,
    "name":"Test Writer 2"
  }
]

Links:

[
  {
    "relationship":"Teacher/student",
    "source":73, //These numbers align to the ids and index of the nodes
    "target":74
  },
  {
    "relationship":"Teacher/student",
    "source":73,
    "target":75
  }
]

The simplest solution, I think, would be to add link data to the node JSON, like the Google group comment suggested. However, I don't think I can do that with the way the database I'm drawing from is set up (I am really bad at SQL though, so I'm not positive that's true.). I also don't have permission to edit the database; I can only select.

Here's the current state of the adjacent node code, which doesn't really do anything at all:

        var hash_lookup = [];
        nodesData.forEach(function(d, i) {
          hash_lookup[d.id] = d;
        });
        linksData.forEach(function(d, i) {
          d.source = hash_lookup[d.source];
          d.target = hash_lookup[d.target];
        });


        function neighboring(a, b) {
          return hash_lookup[a.index + "," + b.index];
        }

And here's a fiddle.

I'm new to coding, so I'm not sure if I'm making a stupid mistake somewhere. I've been working on this for the better part of a week and haven't made any progress on my own, though. Any help is appreciated.

Edit:

Here's another version of the code, adapted from Mike Bostock's solution:

var linkedByIndex = {};
linksData.forEach(function(d) {
  linkedByIndex[d.source.index + "," + d.target.index] = 1;
});

function neighboring(a, b) {
  return linkedByIndex[a.index + "," + b.index];
}

node.on("mouseover.four", function () {
    node.style("opacity", function(d,o) {
  return neighboring(d, o) ? 1 : 0.3;
});
});

Edit 2:

Here's the solution. You have to put the code after the force function is called or it won't work:

force
            .nodes(nodesData)
            .links(linksData)
            .on("tick", tick)
            .start();

node.on("mouseover", fade(.1))
    .on("mouseout", fade(1));

var linkedByIndex = {};

linksData.forEach(function(d) {
  linkedByIndex[d.source.index + "," + d.target.index] = 1;
});

function isConnected(a, b) {
    return linkedByIndex[a.index + "," + b.index] || linkedByIndex[b.index + "," + a.index] || a.index == b.index;
}

function tick() {
//code code code//
}

    function fade(opacity) {
        return function(d) {
            node.style("stroke-opacity", function(o) {
                thisOpacity = isConnected(d, o) ? 1 : opacity;
                this.setAttribute('fill-opacity', thisOpacity);
                return thisOpacity;
            });

            link.style("stroke-opacity", opacity).style("stroke-opacity", function(o) {
                return o.source === d || o.target === d ? 1 : opacity;
            });
        };
    };