firebase.service.ts

import { AngularFirestore, QueryFn } from '@angular/fire/firestore';
import { Injectable } from '@angular/core';
import { Observable } from 'rxjs-compat';
import { map } from 'rxjs/operators';
import * as firebase from 'firebase';

@Injectable()
export class FirebaseService<T> {

  constructor(
    private afs: AngularFirestore,
  ) { }

  public add(path: string, payload: T): Promise<firebase.firestore.DocumentReference> {
    payload['createdAt'] = firebase.firestore.FieldValue.serverTimestamp()
    return this.afs.collection(path).add(payload);    
  }
  
  public get(path: string, queryFn?: QueryFn): Observable<T[]> {
    const collection =  this.afs.collection<any>(path, queryFn);
    return collection.snapshotChanges().pipe(
      map(actions => {
        return actions.map(a => {
          const data = a.payload.doc.data();
          const id = a.payload.doc.id;
          return { id, ...data } as any;
        });
      }),
    );
  }

  public getDoc(path: string): Observable<T> {
    const document = this.afs.doc(path);
    return document.snapshotChanges().pipe(
      map(action => {
        const data = action.payload.data();
        const id = action.payload.id;
        return { id, ...data } as any;
      }),
    );
  }

  public update(path: string, payload: T): Promise<void> {
    const doc = this.afs.doc(path);
    return doc.update(payload);
  }

}