How do you do an update?

[codegrue]

When you map a subscription to an ObservableCollection of POCO objects, they don't have a property for the "key". How would you get the relevant key to perform an update back in firebase?

        var changedTodo = observableCollection.First();
        changedTodo.Completed = true;
        changedTodo.Description = "Modified Text Here";

        var key = ????

        todosRealTimeDb.Put(key, changedTodo);