code: function ViewModel (attr, repository, version) {
    if (!repository) {
      var errMsg = 'Please pass in a valid repository';
      console.log(errMsg);
      throw new Error(errMsg);
    }
  
    for (var f in Repository.prototype) {
      if (!_.isFunction(repository[f])) {
        var errMsg = 'Please pass in a valid repository';
        console.log(errMsg);
        throw new Error(errMsg);
      }
    }
  
    if (attr.id) {
      this.id = _.clone(attr.id);
    }
  
    // version addition, used by elasticsearch6, null means a new model
    if (version !== undefined)
      this.version = _.clone(version);
  
    this.actionOnCommit = 'create';
    this.repository = repository;
  
    this.attributes = _.cloneDeep(attr);
  }
docs: ViewModel constructor
  @param {String} attr The attributes of this viewmodel instance.