How to return true or false if a document exists in Angular Firebase?

Question

I am currently working on a project that requires angular as a front end and firebase as a backend. However, when I was querying for a doc, I can't do something in a snapshot but can't seem to return anything.

this.afs.doc(`meals/${meal.id}`).collection('pending', ref => ref.where('uid', '==', this.authService.getUserID())).valueChanges().pipe(take(1)).subscribe(matchingRecs => {
  if(matchingRecs.length > 0) {
    return true;
  }
  else return false
})

I have this at the moment and it returns underfined but i know its true because the console log prints true

Does this answer your question? [Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference](https://stackoverflow.com/questions/23667086/why-is-my-variable-unaltered-after-i-modify-it-inside-of-a-function-asynchron) — John Montgomery, Mar 30 '20 at 23:09

score 0 · Answer 1 · answered Mar 30 '20 at 23:10

0

try to include into pipe a map operator

pipe(map(matchingRecs => matchingRecs.length > 0))

then in subscribe u will get only boolean values. also you can't return something from subscribe, you need to write logic inside subscribe callback

answered Mar 30 '20 at 23:10

Vyacheslav Zhabitsky

354
3
11

julianobrasil · Answer 2 · 2020-03-31T01:33:57.140

0

You're returning something in the subscriber function and its not supposed to return anything. If you need to return something, you should subscribe on the variable that receives the return and use a map instead of a subscribe:

let result!: boolean = this.afs.doc(`meals/${meal.id}`)
  .collection('pending', ref => ref.where(
    'uid', '==', this.authService.getUserID())
  ).valueChanges().pipe(
    map(matchingRecs => !!matchingRecs.length),
    take(1)
  );

result.subscribe(console.log);

You can use a get instead of valueChanges to get a single value:

let result!: boolean = this.afs.doc(`meals/${meal.id}`)
  .collection('pending', ref => ref.where(
    'uid', '==', this.authService.getUserID())
  ).get().pipe(
    map((v: firestore.QuerySnapshot<firestore.DocumentData>) => !!v.docs.length)
  );

result.subscribe(console.log);

edited Mar 31 '20 at 01:33

answered Mar 30 '20 at 23:15

julianobrasil

8,954
2
33
55

So far me, the result variable is giving me an error saying that an obervable cannot be assined to type boolean – Aaron Vo Mar 31 '20 at 01:11
I fixed the problem. – julianobrasil Mar 31 '20 at 01:34
I still can't seem to return anything. The thing is, I'm specifically calling this method from somewhere else and I want to be able to return true of a doc exist and not if it doesnt; – Aaron Vo Mar 31 '20 at 07:34
Return result and subscribe to the returned value. An observable doesn't return values, just observables. – julianobrasil Mar 31 '20 at 11:16

Segev -CJ- Shmueli · Answer 3 · 2020-03-31T13:13:26.250

I would attack a bit differently altogether, but building on your query the easiest change would be :

return this.afs.doc(`meals/${meal.id}`)
      .collection('pending', ref => ref.where('uid', '==', 
          this.authService.getUserID()))
      .valueChanges()
      .pipe(switchMap(docs => of(docs.length > 0)));

-- or --

return this.afs.doc(`meals/${meal.id}`)
      .collection('pending', ref => ref.where('uid', '==', 
          this.authService.getUserID()))
      .valueChanges()
      .pipe(map(docs => docs.length > 0));

-- or if you need to return a promise an observable --

async myFunc() : Promise<boolean> {
    const hasRecords = await this.afs.doc(`meals/${meal.id}`)
      .collection('pending', ref => ref.where('uid', '==', 
          this.authService.getUserID()))
      .valueChanges()
      .pipe(map(docs => docs.length > 0)).toPromise();

    return hasRecords;
}

This is only returning an object array rather than a boolean — Aaron Vo, Mar 31 '20 at 07:30
please read the code again. It returns the result of `docs.length > 0`, a boolean — Segev -CJ- Shmueli, Mar 31 '20 at 13:11

How to return true or false if a document exists in Angular Firebase?

3 Answers3