Are you sure filterBy doesn't return an array? so something like [<App.Topology:ember411:1>]
Because that's what it's supposed to do, see http://emberjs.com/api/classes/Ember.Enumerable.html#method_filterBy. If it does return an array you can use the firstObject property:
return this.filterBy('name', this.get('selectedName')).get('firstObject').get('site');