I use something similar like this. In my case I don´t use Part, so I am not sure if this one would work in reality. But I hope somebody can help me to find the missing piece:
var myItem = aras.IomInnovator.getItemById("Part",this.getProperty("id"));
myItem.setProperty("description","i am new");
var relCosts = myItem.getRelationships("Part Goal");
for (var i = 0; i < relCosts.getItemCount(); i++)
var singleGoal = relCosts.getItemByIndex(i);
myItem = myItem.apply();
Thanks already for every ideas!