AngularFire2 promises into Observable type Issues

Question

I'm trying to map angularfire2's promises into typed observables. This is my class:

export class RestService<T extends Entity> {

    constructor(
        protected afAuth: AngularFireAuth,
        protected store: AngularFirestore,
        protected readonly collectionName: string) {

        this.collection = store.collection<T>(collectionName);

    }

    protected collection: AngularFirestoreCollection<any>;

    all(): Observable<Array<T>> {
        return from(this.collection
            .snapshotChanges()
            .pipe(
                map(action => mapActionArray(action)),
                catchError((error: any) => {
                    throw new ApiError(error);
                })
            )
        );
    }


}

/**
 * Override Firestore errors with common app errors.
 * @param error - Firestore error
 */
function mapError(error: any): Observable<ApiError> {
    return throwError(new ApiError(error));
}

function mapDocument<T extends Entity>(doc: DocumentSnapshot<any>): T {
    return {
        id: doc.id,
        ...doc.data()
    };
}

Notice the all() method, I want to map any error in the catchError, but the thing is I get this type error:

TS2322: Type 'Observable<{}[]>' is not assignable to type 'Observable'.   Type '{}[]' is not assignable to type 'T[]'.     Type '{}' is not assignable to type 'T'.

I fixed it by changing the return type to Observable<{} | Array>, but that would require using the same types whenever I use this service, I don't find it good. How do I work around this? I want to be able to type my services and in beforehand map the errors into a proper CustomError object (this includes mapping codes to messages).

Btw, I also tried using the throwError from RXJS.

Thanks in advance.

Answer 1

Answering my own question, I had to use the catchError before the map operator. So:

  all(): Observable<Array<T>> {
    return from(this.collection
        .snapshotChanges()
        .pipe(
            map(action => mapActionArray(action)),
            catchError((error: any) => {
                throw new ApiError(error);
            })
        )
    );
}

Would become

  all(): Observable<Array<T>> {
    return from(this.collection
        .snapshotChanges()             
        .pipe(
            catchError((error: any) => {
                throw new ApiError(error);
            }),
            map(action => mapActionArray(action))                
        )
    );
}

Hope it helps