angular - return FirebaseObjectObservable from Observable.mergeMap -

i'm creating abstract data service class application, have abstract method getbaseuri returns observable<string> (because need user id in uri, meaning has observable).

the issue because base oservable observable, final observable of list observable<t>, not firebaseobjectobservable <t>.

here i'm using:

the abstract method:

protected abstract getbaseuri(): observable<string>;

my listref getter:

private oneref(uid: string): firebaseobjectobservable <t> {     return this.getbaseuri().mergemap(uri => this.firebase.object(`${uri}/${uid}`)); } 

the current runtime error is:

error typeerror: this.oneref(...).update not function.