using underscore's “difference” method on arrays of objects

_.difference([], [])

this method works fine when i'm having primitive type data like

var a = [1,2,3,4];
var b = [2,5,6];

and the _.difference(a,b) call returns [1,3,4]

but in case i'm using object like

var a = [{'id':1, 'value':10}, {'id':2, 'value':20}];
var b = [{'id':1, 'value':10}, {'id':4, 'value':40}];

doesn't seem to work