iD.Relation = iD.Entity.extend({
    type: "relation",
    members: [],

    extent: function() {
        return [[NaN, NaN], [NaN, NaN]];
    }
});

@@ -132,16 +132,78 @@
  NaN%5D%5D;%0A
+    %7D,%0A%0A    geometry: function() %7B%0A        return 'relation';%0A
     %7D%0A%7D)

Define Relation#geometry