Odtnhj

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.

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; 
 
 );
 

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.