Sequential and dependent queries with rxjs

Question

I'm trying to make multiple sequential and dependent queries on a Firebase database. This is how the database look like :

books : { book1: { title: 'Book 1', author: 'author1' } }
authors: { author1: { name: 'Author 1' } }

I'd like to retrieve all the books, iterate through them, get the related author name, set the value in each book object and return an Observable array containing the book objects.

I tried this way :

getBooks(): Observable<Book[]> {
    return this._af.database
        .list('/books')
        .flatMap(e => e)
        .concatMap(e => this.getAuthor(e.author), (a, b) => {
            a.author = b.lastName;
            return a;
        });
}

But this returns the first book only.

Can you be more specific? I tried to use flatMap(), concatMap(), mergeMap(), etc. but, apparently, I don't know how to use them. — untemps
– untemps, Commented Nov 23, 2016 at 10:02
Can you be more specific? ;-) Please edit the question and post the code that demonstrates what you try to accomplish, what you tried and where you failed. — Günter Zöchbauer
– Günter Zöchbauer, Commented Nov 23, 2016 at 10:03
@untemps One of these answers could help you stackoverflow.com/questions/40170157/… stackoverflow.com/questions/40346728/… stackoverflow.com/questions/40250882/… — martin
– martin, Commented Nov 23, 2016 at 13:36

Mark van Straten · Accepted Answer · 2016-11-24 10:59:31Z

0

Given that list(/books) returns an array like [bookObj, bookObj] you would need to fetch the author information of each book, no need for concatMap unless order preservation of your input books is required for the output.

  getBooks(): Observable<Book[]> {
    return this._af.database
      .list('/books')
      .flatMap(booksArr => Rx.Observable.from(booksArr))
      .flatMap(
        book => this.getAuthor(e.author),
        (book,a) => { 
          a.author = book.lastName; 
          return a; 
        }
      );
  }

edited Nov 24, 2016 at 10:59

answered Nov 24, 2016 at 7:39

Mark van Straten

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2 Comments

untemps Over a year ago

Thanks but that actually doesn't work since books is an array with no flatMap method.

Mark van Straten Over a year ago

ahh, if list('/books') returns a vanilla array then you need to convert it to an observable using flatMap(booksArr => Rx.Observable.from(booksArr)) so all the books are emitted as events on the observable stream.

untemps · Accepted Answer · 2016-11-24 14:11:14Z

Being unable to achieve this using operators, I have created an Observable via create() method and perform the queries before returning a new array :

getBooks(): Observable<Book[]> {
    return Observable.create(observer => {
        const res: Book[] = [];
        this._af.database
            .list('/books').subscribe(books => {
            books.map(book => {
                this.getAuthor(book.author).subscribe(author => {
                    res.push(
                        new Book(book.$key, book.title, author.lastName, book.price, book.image)
                    );
                    if (res.length === books.length) {
                        observer.next(res);
                    }
                })
            });
        });
    });
}

That's not what I expected but it works.

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