code: function () {
              if (!this.dbReadySubject) {
                  // first time the service is called.
                  this.dbReadySubject = new rxjs.BehaviorSubject(false);
                  this.resetDb();
              }
              return this.dbReadySubject.asObservable().pipe(operators.first(function (r) { return r; }));
          }
docs: //  protected /////