The setter is invalid, you are setting todo
to undefined.
//init at start
controller.set('todo', todoDB.Todos.attachOrGet({ Id:1}));
http://emberjs.jsbin.com/AyIMIBi/1/edit
With remove and completed
http://emberjs.jsbin.com/AyIMIBi/2/edit
Additionally, tho unnecessary, if you were trying to grab the todo off the controller, you should use a getter.
controller.get('todo').save();