How to add a new record (in Ember.js) with belongsTo relationship, check if the belongsTo record exist and add that too if it doesn't.

I have two models: Post and Date.

App.Post = DS.Model.extend({
  date: DS.belongsTo('date', {async: true})
});

App.Date = DS.Model.extend({
  when: DS.attr('date'),    
  posts: DS.hasMany('post', {async: true}),
});

While creating a new post, I need to check if the date user entered already exists in the Date model. If not, a new Date record also needs to be created.

Adding a new post is happening in AddController. I am guessing AddRoute needs both models (post and date) and both controllers set up as follows. Struggling with how to access controllers and models in the AddController (commented below 1,2,3)

App.AddRoute = Ember.Route.extend({
    model: function() {
        return Ember.RSVP.hash({ 
            post: this.store.createRecord('post'),
            dates: this.store.findAll('date')
        });
    },
    setupController: function(controller, model) {
        controller.set('model', model.post);
        controller.set('dates', model.dates);
    }
});

App.AddController = Ember.Controller.extend({
    d:'', //the date user inputs 
    actions: {
        createPost: function() {
            var controller = this;
            console.log(controller.get('d')); 
            controller.get('model').save().then(function(){
//       1. How to check if date already exists? Something like:
//          this.store.find('date', controller.get('d'));
//       2. If date already exists, how to addObject to posts?
//          controllerOfDate.get('model.posts').addObject('post');
//          else (if it's a new date)
//       3. How to create new record in date:  
//          this.store.createRecord('date', {when: controller.get('d')});
//          controllerOfDate.get('model.posts').addObject('post');

       });
      }
    }
    });