Firestore how to get the collection value from another collection document id is referenced

Question

I have two fire store collection with following reference image . I want to get the firstName and title. Here signup_id is referenced from col

Answer 1

For anyone experiencing issues, combining documents in Firebase Cloud Firestore while using Angular6, RxJS 6 and AngularFire v5.0 try the following code

Model feed.ts

export interface Feed {
  firstName?: string;
  signup_id?: string;
  title?: string;
}

export interface CollSignup {
  firstName: string;
  mob: string;
}

export interface ColChallange {
  signup_id: string;
  title: string;
}

Service feed.service.ts

import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/firestore';
import { Observable, combineLatest } from 'rxjs';
import {flatMap, map} from 'rxjs/operators';
import {ColChallange, CollSignup, Feed} from '../../models/feed';

@Injectable()
export class FeedService {
  colChallangeCollection: AngularFirestoreCollection<ColChallange>;
  feedItem: Observable<Feed[]>;

  constructor(private afs: AngularFirestore) { }

  collectionInitialization() {
    this.colChallangeCollection = this.afs.collection('col-challange');
     this.feedItem = this.colChallangeCollection.snapshotChanges().pipe(map(changes  => {
      return changes.map( change => {
        const data = change.payload.doc.data();
        const signupId = data.signup_id;
        const title = data.title;
          return this.afs.doc('coll-signup/' + signupId).valueChanges().pipe(map( (collSignupData: CollSignup) => {
            return Object.assign(
              {firstName: collSignupData.firstName, signup_id: signupId, title: title}); }
          ));
      });
    }), flatMap(feeds => combineLatest(feeds)));
  }

  sellectAllNews() {
    this.collectionInitialization();
    return this.feedItem;
  }
}

To print all the data

this.feedItem.forEach(value => {
  console.log(value);
});

Answer 2

You need to combine two collections.

try this code

this.feedCollection = this.afs.collection('col-challange');
this.feedItem = this.feedCollection.snapshotChanges().map(changes => {
      return changes.map(a => {
        //here you get the data without first name
        const data = a.payload.doc.data() as Feed;
        //get the signup_id for getting doc from coll-signup
        const signupId = data.signup_id;
        //get the related document
        return afs.collection('coll-signup').doc(signupId).snapshotChanges().take(1).map(actions => {
          return actions.payload.data();
        }).map(signup => {
          //export the data in feeds interface format
          return { firstName: signup.firstName, ...data };
        });
      })
    }).flatMap(feeds => Observable.combineLatest(feeds));