The Ember.js REST Adapter expects the JSON to be returned as:
{
\"person\": {
\"first_name\": \"Barack\",
\"last_name\": \"Obama\",
The easiest way is to not use the RESTSerializer but the much simpler JSONSerializer, which does not expect a root element.
Good resources on understanding which serializer to use for a given API can be found in these two blog posts:
http://thejsguy.com/2015/12/05/which-ember-data-serializer-should-i-use.html
http://emberigniter.com/fit-any-backend-into-ember-custom-adapters-serializers/