D3 drag error 'Cannot read property x of undefined' - d3.js

Question

I'm trying to get drag functionality to work on D3, and have copied the code directly from the developer's example.
However it seems the origin (what is being clicked) is not being passed correctly into the variable d, which leads to the error: 'Cannot read property 'x' of undefined'
The relevant code:
var drag = d3.behavior.drag()
.on("drag", function(d,i) {
d.x += d3.event.dx
d.y += d3.event.dy
d3.select(this).attr("transform", function(d,i){
return "translate(" + [ d.x,d.y ] + ")"
})
});
var svg = d3.select("body").append("svg")
.attr("width", 1000)
.attr("height", 300);
var group = svg.append("svg:g")
.attr("transform", "translate(10, 10)")
.attr("id", "group");
var rect1 = group.append("svg:rect")
.attr("rx", 6)
.attr("ry", 6)
.attr("x", 5/2)
.attr("y", 5/2)
.attr("id", "rect")
.attr("width", 250)
.attr("height", 125)
.style("fill", 'white')
.style("stroke", d3.scale.category20c())
.style('stroke-width', 5)
.call(drag);

Answer 1

Usually, in D3 you create elements out of some sort of datasets. In your case you have just one (perhaps, one day you'll want more than that). Here's how you can do it:
var data = [{x: 2.5, y: 2.5}], // here's a dataset that has one item in it
rects = group.selectAll('rect').data(data) // do a data join on 'rect' nodes
.enter().append('rect') // for all new items append new nodes with the following attributes:
.attr('x', function (d) { return d.x; })
.attr('y', function (d) { return d.y; })
... // other attributes here to modify
.call(drag);
As for the 'drag' event handler:
var drag = d3.behavior.drag()
.on('drag', function (d) {
d.x += d3.event.dx;
d.y += d3.event.dy;
d3.select(this)
.attr('transform', 'translate(' + d.x + ',' + d.y + ')');
});

Answer 2

Oleg's got it, I just wanted to mention one other thing you might do in your case.
Since you only have a single rect, you can bind data directly to it with .datum() and not bother with computing a join or having an enter selection:
var rect1 = svg.append('rect')
.datum([{x: 2.5, y: 2.5}])
.attr('x', function (d) { return d.x; })
.attr('y', function (d) { return d.y; })
//... other attributes here
.call(drag);