shangyilim

// Retrieve cities from California
db.collection("cities").where("state", "==", "CA")
  .onSnapshot(function(querySnapshot) {
        querySnapshot.forEach(function(doc) {
            // binded to the UI
            allItems.push(doc.data().name);
        });
    });
// Retrieve cities from Colorado

shangyilim

<!-- list of California cities goes here -->
<ul>
   ...
</ul>
<!-- list of Colerado cities goes here -->
<ul
   ...
</ul>

shangyilim

// Query for California
var californiaRef = citiesRef.where("state", "==", "CA");

// Query for Colorado
var coloradoRef = citiesRef.where("state", "==", "CO");

// Create Observables.
var california$ = new Rx.Subject();
var colorado$ = new Rx.Subject();

shangyilim

// When either California OR Colorado values are emitted from their queries,
// combine both observables 
var californiaOrColorado$ = Rx.Observable.combineLatest(california$, colorado$).switchMap((cities) => {
    // Destructure the values to combine a single array.
    var [californiaCities, coloradoCities] = cities;
    var combined = [
        //spread the arrays out to combine as one array
        ...californiaCities,
        ...coloradoCities
    ]

shangyilim

// Subscribe to the latest stream that contains both 
// California and Colorado.
californiaOrColorado$.subscribe((result) => {
  // Output to UI.
  var citiesEle = document.getElementById('cities');
  result.forEach(city => {
      var itemEle = document.createElement("li");
      itemEle.textContent = city.name;
      citiesEle.appendChild(itemEle);
    });

shangyilim

caOrCoCities$: Observable<City[]>;
...
const californiaRef = this.angularFirestore
    .collection("cities", ref => ref.where("state","==","CA"));
const coloradoRef = this.angularFirestore
    .collection("cities", ref => ref.where("state","==","CO"));
this.caOrCoCities$ = Observable
    .combineLatest(californiaRef.valueChanges(),
                   coloradoRef.valueChanges())
    .switchMap(cities => {

shangyilim

<ul>
   <li *ngFor="let cities of caOrCoCities$ | async as caOrCoCities">
     ...
   </li>
</ul>

shangyilim

import { collection } from 'rxfire/firestore';
import { map } from 'rxjs/operators';

// Query for California
var californiaRef = citiesRef.where("state", "==", "CA");

// Query for Colorado
var coloradoRef = citiesRef.where("state", "==", "CO");

// Create Observables ready for subscribe().

shangyilim

import { combineLatest } from 'rxjs';

var californiaOrColorado$ = combineLatest(california$, colorado$)
.pipe(
  map((cities) => {
    // Destructure the values to combine a single array.
    var [californiaCities, coloradoCities] = cities;
    return [
        //spread the arrays out to combine as one array
        ...californiaCities,

shangyilim

import * as functions from "firebase-functions";
import * as admin from "firebase-admin";
const firebase = admin.initializeApp();

export const deletePhotos = functions.firestore
  .document("posts/{postId}")
  .onDelete((snap, context) => {
    const { postId } = context.params;
    const bucket = firebase.storage().bucket();