products.component.ts

this.products = this.productService.products().snapshotChanges().map(productSnaps => {
      return productSnaps.map(product => {
        const productData = product.payload.doc.data();
        const productId = product.payload.doc.id;
        return this.productService.getProductImages(productId).snapshotChanges().map(uploadSnap => {
          let number = 0;
          return uploadSnap.map(upload => {
            if(number == 0) {
              number++;
              return upload.payload.doc.data();
            }
          })
        })
        .map(uploads => {
          return {productId, ...productData, uploads: uploads};
        })
      })
    })
    .flatMap(products => Observable.combineLatest(products));