Ember: ASync belongs to doesn't seem to resolve

I have the following two models:

models/profile.js

import DS from 'ember-data';

export default DS.Model.extend({
  firstName: DS.attr('string'),
  lastName: DS.attr('string')
});

models/user.js

import DS from 'ember-data';

export default DS.Model.extend({
  profile: DS.belongsTo('profile', {async:true})
});

The user models is loaded by a user service, based on a cookie set by the server when the user authenticates:

services/user.js

import Ember from 'ember';

export default Ember.Service.extend({
    store: Ember.inject.service('store'),
    init: function() {
        var _this = this;
        _this.set('user', null)
        if(Cookies.get('uid')) {
            _this.get('store').findRecord('user', Cookies.get('uid')).then(function(user) {
                console.log(user)
                _this.set('user', user)
            }, function(err) {
                console.warn(err);
                Cookies.remove('uid')
                _this.set('user', null)
            });
        } else {
            _this.set('user', null)
        }
    }
});

The user service is injected into the application controller:

controllers/applications.js

import Ember from 'ember';

export default Ember.Controller.extend({
    user: Ember.inject.service('user')
});

I have some debugging output in my main template

templates/application.hbs

{{#if user.user}}
    <h2 id="title">Welcome to Ember {{user.user}} # {{user.user.id}} # {{user.user.profile}} # {{user.user.profile.firstName}}</h2>
{{else}}
    <h2 id="title">Please Log In <a href="/auth/linkedin">Linkedin</a></h2>
{{/if}}

{{outlet}}

which when I authenticate and load the page outputs

Welcome to Ember <project@model:user::ember418:5663bfe19d78ce048784a098> # 5663bfe19d78ce048784a098 # <DS.PromiseObject:ember419> #

I can see requests made to my api endpoints in the console, first for the user then for the profile, as expected:

http://localhost:3000/api/users/5663bfe19d78ce048784a098

{
    links: {
        self: "http://localhost:3000/api/users/5663bfe19d78ce048784a098"
    },
    data: {
        id: "5663bfe19d78ce048784a098",
        type: "users",
        attributes: {
            providers: [{
                provider: "linkedin",
                id: "ECixxxxqc9",
                displayName: "John Smith",
                name: {
                    familyName: "Smith",
                    givenName: "John"
                }
            }]
        },
        links: {
            self: "/users/5663bfe19d78ce048784a098"
        },
        relationships: {
            profile: {
                data: {
                    type: "profiles",
                    id: "5663c05f5fd3331387307e89"
                }
            }
        }
    }
}

http://localhost:3000/api/profiles/5663c05f5fd3331387307e89

{
    links: {
        self: "http://localhost:3000/api/profiles/5663c05f5fd3331387307e89"
    },
    data: {
        id: "5663c05f5fd3331387307e89",
        type: "profiles",
        attributes: {
            active: true,
            yearsExperience: 9,
            lastName: "Smith",
            firstName: "John"
        },
        links: {
            self: "/profiles/5663c05f5fd3331387307e89"
        }
    }
}

It seems like the user.user.profile promise isn't being resolved, hence the <DS.PromiseObject:ember419> output and no value for user.user.profile.firstName. Is there something I'm missing in the model definitions or is there something I need to call to manually tell the controller/template that a belongsTo has been resolved?