How to handle an observable that may return empty when using switchMap RxJS










1















how should the following scenario be handled with RxJS?



I have an observable that queries my Firestore database for a submission by an ID extracted from the url parameters:



this.submission$ = this.db.collection$('submissions', ref => ref.where('submissionArtistUID', '==', this.artistParam));


For elements subscribed to this observable the submission$ returns an array of submission objects. Thus I can use switchMap to query my users table once the submission$ has resolved:



this.user$ = this.submission$.pipe(
switchMap(
(submissions) =>
return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

)
)


However, the submissions query is not guaranteed to return anything. So when I used the switchMap on a submission that returns empty, I cannot access the following property: submissions[0].submissionUserUID, and I'm shown this error in console:



Cannot read property 'submissionUserUID' of undefined



Which makes sense because the submission$ resolves to an empty object before trying to be used in the return statement of the switchMap.



So how do I account for scenario like this using RxJS? Is switchMap the right thing to use? Thanks!










share|improve this question


























    1















    how should the following scenario be handled with RxJS?



    I have an observable that queries my Firestore database for a submission by an ID extracted from the url parameters:



    this.submission$ = this.db.collection$('submissions', ref => ref.where('submissionArtistUID', '==', this.artistParam));


    For elements subscribed to this observable the submission$ returns an array of submission objects. Thus I can use switchMap to query my users table once the submission$ has resolved:



    this.user$ = this.submission$.pipe(
    switchMap(
    (submissions) =>
    return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

    )
    )


    However, the submissions query is not guaranteed to return anything. So when I used the switchMap on a submission that returns empty, I cannot access the following property: submissions[0].submissionUserUID, and I'm shown this error in console:



    Cannot read property 'submissionUserUID' of undefined



    Which makes sense because the submission$ resolves to an empty object before trying to be used in the return statement of the switchMap.



    So how do I account for scenario like this using RxJS? Is switchMap the right thing to use? Thanks!










    share|improve this question
























      1












      1








      1








      how should the following scenario be handled with RxJS?



      I have an observable that queries my Firestore database for a submission by an ID extracted from the url parameters:



      this.submission$ = this.db.collection$('submissions', ref => ref.where('submissionArtistUID', '==', this.artistParam));


      For elements subscribed to this observable the submission$ returns an array of submission objects. Thus I can use switchMap to query my users table once the submission$ has resolved:



      this.user$ = this.submission$.pipe(
      switchMap(
      (submissions) =>
      return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

      )
      )


      However, the submissions query is not guaranteed to return anything. So when I used the switchMap on a submission that returns empty, I cannot access the following property: submissions[0].submissionUserUID, and I'm shown this error in console:



      Cannot read property 'submissionUserUID' of undefined



      Which makes sense because the submission$ resolves to an empty object before trying to be used in the return statement of the switchMap.



      So how do I account for scenario like this using RxJS? Is switchMap the right thing to use? Thanks!










      share|improve this question














      how should the following scenario be handled with RxJS?



      I have an observable that queries my Firestore database for a submission by an ID extracted from the url parameters:



      this.submission$ = this.db.collection$('submissions', ref => ref.where('submissionArtistUID', '==', this.artistParam));


      For elements subscribed to this observable the submission$ returns an array of submission objects. Thus I can use switchMap to query my users table once the submission$ has resolved:



      this.user$ = this.submission$.pipe(
      switchMap(
      (submissions) =>
      return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

      )
      )


      However, the submissions query is not guaranteed to return anything. So when I used the switchMap on a submission that returns empty, I cannot access the following property: submissions[0].submissionUserUID, and I'm shown this error in console:



      Cannot read property 'submissionUserUID' of undefined



      Which makes sense because the submission$ resolves to an empty object before trying to be used in the return statement of the switchMap.



      So how do I account for scenario like this using RxJS? Is switchMap the right thing to use? Thanks!







      angular rxjs angularfire2






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      share|improve this question











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      asked Nov 13 '18 at 4:17









      Jordan LewallenJordan Lewallen

      479311




      479311






















          1 Answer
          1






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          oldest

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          3














          You can use filter operator before switchmap. Your code might look like this.



          this.user$ = this.submission$.pipe(
          filter(submissions => Object.keys(submissions).length > 0),
          switchMap(
          (submissions) =>
          return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

          )
          )





          share|improve this answer

























          • very concise, filter did the trick! Thanks for teaching me an application of that operator :)

            – Jordan Lewallen
            Nov 13 '18 at 4:53











          • Awesome. Glad to help

            – KiraAG
            Nov 13 '18 at 5:00










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          1 Answer
          1






          active

          oldest

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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          3














          You can use filter operator before switchmap. Your code might look like this.



          this.user$ = this.submission$.pipe(
          filter(submissions => Object.keys(submissions).length > 0),
          switchMap(
          (submissions) =>
          return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

          )
          )





          share|improve this answer

























          • very concise, filter did the trick! Thanks for teaching me an application of that operator :)

            – Jordan Lewallen
            Nov 13 '18 at 4:53











          • Awesome. Glad to help

            – KiraAG
            Nov 13 '18 at 5:00















          3














          You can use filter operator before switchmap. Your code might look like this.



          this.user$ = this.submission$.pipe(
          filter(submissions => Object.keys(submissions).length > 0),
          switchMap(
          (submissions) =>
          return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

          )
          )





          share|improve this answer

























          • very concise, filter did the trick! Thanks for teaching me an application of that operator :)

            – Jordan Lewallen
            Nov 13 '18 at 4:53











          • Awesome. Glad to help

            – KiraAG
            Nov 13 '18 at 5:00













          3












          3








          3







          You can use filter operator before switchmap. Your code might look like this.



          this.user$ = this.submission$.pipe(
          filter(submissions => Object.keys(submissions).length > 0),
          switchMap(
          (submissions) =>
          return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

          )
          )





          share|improve this answer















          You can use filter operator before switchmap. Your code might look like this.



          this.user$ = this.submission$.pipe(
          filter(submissions => Object.keys(submissions).length > 0),
          switchMap(
          (submissions) =>
          return this.db.collection$('users', ref => ref.where('userUID', '==', submissions[0].submissionUserUID ))

          )
          )






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Nov 13 '18 at 6:21









          CozyAzure

          5,37841836




          5,37841836










          answered Nov 13 '18 at 4:44









          KiraAGKiraAG

          391110




          391110












          • very concise, filter did the trick! Thanks for teaching me an application of that operator :)

            – Jordan Lewallen
            Nov 13 '18 at 4:53











          • Awesome. Glad to help

            – KiraAG
            Nov 13 '18 at 5:00

















          • very concise, filter did the trick! Thanks for teaching me an application of that operator :)

            – Jordan Lewallen
            Nov 13 '18 at 4:53











          • Awesome. Glad to help

            – KiraAG
            Nov 13 '18 at 5:00
















          very concise, filter did the trick! Thanks for teaching me an application of that operator :)

          – Jordan Lewallen
          Nov 13 '18 at 4:53





          very concise, filter did the trick! Thanks for teaching me an application of that operator :)

          – Jordan Lewallen
          Nov 13 '18 at 4:53













          Awesome. Glad to help

          – KiraAG
          Nov 13 '18 at 5:00





          Awesome. Glad to help

          – KiraAG
          Nov 13 '18 at 5:00

















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