A guide on efficiently utilizing combineLatest and mergeMap for handling multiple subscriptions

As I continue to delve into the world of rxjs, I've encountered an issue with managing multiple subscriptions. Specifically, I'm struggling to extract an ID from a response in order to correctly associate photos with products.

create(product) {
     this.api.createProduct(product).subscribe(product => {
       product.photos.forEach(photo => {
         photo.productId = product.id;
         this.api.addPhoto(photo).subscribe();
      });
    });

While my current solution is functional, I understand that it's not the most efficient approach. I've heard that utilizing mergeMap and combineLatest could be more effective, but I'm unsure how to implement them properly.

Update 1

Improved solution

create(productForm) {
  this.api.createProduct(productForm).pipe(
    mergeMap(photo => {
      return combineLatest([
        productForm.photos.map(photo => {
          this.api.addPhoto({...photo, productId: product.id})//.subscribe()
          })
        ]);
   ).subscribe();
}

Despite this attempted improvement, I still encountered challenges actually adding photos using this method. It only worked when I included subscribe within addPhoto.

Update 2

Revised solution

create(productForm) {
  this.api.createProduct(productForm).pipe(
    mergeMap(photo => {
      const photos = [];
      productForm.photos.map(photo => {
        photos.push(this.api.addPhoto({...photo, productId: product.id}));
      });
      return combineLatest([...photos]);
   })
  ).subscribe();
}

Finally, by making these adjustments, I have managed to successfully achieve the desired functionality for adding photos.

Answer №1

Consider creating a versatile pipeline that can handle multiple tasks efficiently. When the need arises, simply subscribe to it once.

It seems unclear why your forEach loop includes both comment and photo; is this a mistake?

Your workflow could be structured in the following way:

const flow$ = this.api.createProduct(product).pipe(
  mergeMap(product => {
    const sequence = [];
    product.photos.forEach(photo =>
      sequence.push(this.api.addPhoto({...photo, productId: product.id)),
    );
    return forkJoin(...sequence); // <- adding photos after product creation.
  }),
);

// subscription point
flow$.subscribe(); // <- activates the entire pipeline.

Alternatively, you could create an Observable specifically for handling photos and seamlessly integrate it into the main flow.

const addPhotos =(product) => product.photos.map(photo => api.addPhoto({...photo, productId: product.id));

cont flow$ = api.createProduct(product).pipe(
  map(addPhotos),
  mergeMap(o$ => forkJoin(...o$)),
);

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