_route.js

// /store/route.js
import Ember from 'ember';

export default Ember.Route.extend({
  model: function() {
    var orderOptions = {
      type: 'subscription',
      status: '13'
    };

    return Ember.RSVP.hash({
      plans: this.store.find('plan'),
      order: this.store.createRecord('order', orderOptions)
    });
  }
});

order.js

import DS from 'ember-data';
// ember-model-validator
import Validator from '../mixins/model-validator';

var attr = DS.attr;

export default DS.Model.extend(Validator, {
  plan: DS.belongsTo('plan'),
  validations: {
    plan: {
      presence: {
        message: 'Please select a plan'
      }
    }
  }
});

route.js

// /store/checkout/route.js
import Ember from 'ember';

var debug = Ember.Logger.debug;

export default Ember.Route.extend({
  persistOrder: function () {
    debug('persistOrder');
    var order = this.get('currentModel.order');
    var deferred = new Ember.RSVP.defer();
    /*
      if validate fails set order as isNew == false and isDirty == true
      so when validate pass it issue a PUT request instead of a POST request
      even though order doesn't has id, how can I force a POST request?
    */
    if (order.validate()) {
      if (!order.get('id')) {
        return order.save().then(deferred.resolve);
      }
      deferred.resolve();
    } else {
      this.controller.set('orderErrors', order.get('errors.content'));
      deferred.reject();
    }
    return deferred.promise;
  },
  _addToCart: function (currentProduct) {
    var order = this.get('currentModel.order');
    this.persistOrder()
    .then(function() {
      ...
    })
    .catch(function(err) {
      debug('error caught', err);
      throw err;
    });
  },
  actions: {
    addToCart: function addToCart(currentProduct) {
      this._addToCart(currentProduct);
    }
  }
});