TODO: implement for when '.get(relation)' returns a Promise

[Leooo]

I'm not sure in which cases ember doesn't returns promises for relations (it seems to be the standard behaviour with ember-data, even for async=false relationship), but in case it helps this is how I changed your code to work with promises (haven't checked if hasMany relationships work yet):

import RecordForSynchronization from 'ember-sync/record-for-synchronization';
import Ember from 'ember';

var alreadyRun=false;

export default {
  name: 'override-ember-sync',
  initialize: function() {
    if (alreadyRun) {
      return;
    } else {
      alreadyRun=true;
    }

    RecordForSynchronization.reopen({
      toEmberData: function() {
        var _this = this,
            record;

        return new Ember.RSVP.Promise(function(resolve, reject) {
          record = _this.createRecordInStore();
          _this.setRelationships(record).then(function(rec){
            resolve(rec);
          });
        });
      },
      setRelationships: function(pendingRecord) {
        var _this = this,
            offlineRecord = this.get('offlineRecord'),
            type = this.get('jobRecord.jobRecordType');

        /**
         * We need to loop relationships. If no record was
         * passed in, we need to create a new one, fake, so we know about the
         * relationships.
         */
        if (!offlineRecord) {
          offlineRecord = this.onlineStore.push(type, {id: 1});
        }
        var promises=[];
        offlineRecord.eachRelationship(function(name, descriptor) {
          var key = descriptor.key,
              kind = descriptor.kind,
              type = descriptor.type,
              relation, onlineRelation;

          /**
           * TODO - implement for when `.get(relation)` returns a Promise.
           */
          promises.push(offlineRecord.get(name).then(function(relation){
            /**
             * We need to attach relationships to the main record. If the
             * relationships don't exist anymore offline, we need to generate a new
             * one, fake, with the same ID, just to send to the server.
             */
            if (kind == "belongsTo") {
              var relationId = _this.get('jobRecord.serialized')[name];

              if (relationId && !relation) {
                relation = _this.onlineStore.push(type, {id: relationId});
              }

              if (relation) {
                onlineRelation = _this.generateRelationForRecord(type, relation);
                pendingRecord.set(key, onlineRelation);
              }
            } else if (kind == "hasMany") {
              relation.forEach(function(relation) {
                onlineRelation = _this.generateRelationForRecord(type, relation);
                pendingRecord.get(name).pushObject(onlineRelation);
              });
            }
            return pendingRecord;
          }));
        });
        return Ember.RSVP.all(promises).then(function(res){
          return pendingRecord;
        });
      },
    });
  },
}

[kurko]

Any chance you could move this into a PR?