code: function(keys) {
      var missingRelations = [];
      var relationKeys = this.relationDefinitions ? Object.keys(this.relationDefinitions) : [];
      _.each(keys, function(key) {
        var isRelation = relationKeys.indexOf(key) > -1;
        if (isRelation && _.isUndefined(this.get(key))) {
          missingRelations.push(key);
        }
      }, this);
      return _.uniq(missingRelations);
    }
docs: ## BaseModel.prototype.missingRelations
  
  Check missing relations based on keys array.
  @param  {Array} keys array of strings
  @returns {Array} keys of missing relations