attempt to update workaround at http://stackoverflow.com/questions/14685046/how-to-rollback-relationship-changes-in-emberdata/27184207#27184207

model.js

import DS from 'ember-data';
import Ember from 'ember';

export function initialize(/* container, application */) {

    DS.Model.reopen({

        saveOriginalRelations: function() {

            this.originalRelations = {};
            this.constructor.eachRelationship(function(key, relationship) {

                if (relationship.kind === 'belongsTo'){
                    this.originalRelations[key] = this.get(key); //<==== this.get(key); returns always null!!!!
                }
                if (relationship.kind === 'hasMany'){
                    this.originalRelations[key] = this.get(key).toArray();
                }
            }, this);
        },

        onLoad: function() {

            this.saveOriginalRelations();

        }.on('didLoad', 'didCreate', 'didUpdate'),

        onReloading: function() {

            if (!this.get('isReloading')){
                            this.saveOriginalRelations();
            }
        }.observes('isReloading'),    

        rollback: function() {

            this._super();

            if (!this.originalRelations){
                            return;
            }
            Ember.keys(this.originalRelations).forEach(function(key) {

                // careful, as Ember.typeOf for ArrayProxy is 'instance'
                if (Ember.isArray(this.get(key))) {
                    this.get(key).setObjects(this.originalRelations[key]);
                    this.get(key).filterBy('hasDirtyAttributes').invoke('rollback');
                    return;
                }

                if (Ember.typeOf(this.get(key)) === 'instance') {
                    this.set(key, this.originalRelations[key]);
                    return;
                }

            }, this);
        },

        isDeepDirty: function() {
            if (this._super('hasDirtyAttributes')){
                            return true;
            }
            if (!this.originalRelations){
                            return false;
            }
            return Ember.keys(this.originalRelations).any(function(key) {

                if (Ember.isArray(this.get(key))) {
                    if (this.get(key).anyBy('hasDirtyAttributes')){
                                            return true;
                    }
                    if (this.get(key).get('length') !== this.originalRelations[key].length){
                                            return true;
                    }
                    var dirty = false;
                    this.get(key).forEach(function(item, index) {
                        if (item.get('id') !== this.originalRelations[key][index].get('id')){
                                                    dirty = true;}
                    }, this);

                    return dirty;
                }

                return this.get(key).get('hasDirtyAttributes') || this.get(key).get('id') !== this.originalRelations[key].get('id');

            }, this);
        }
    });
}

export default {
    name: 'model',
    initialize: initialize
};

Change this.constructor.eachRelationship to this.eachRelationship (line 11) and this.get will no longer always return null.