如果没有数据，如何处理firebase查询

Question

我正在尝试为父集合中的每个文档获取一个集合及其子集合。编辑过的

  fetchNewMarkets() {
    this.firestore.collection<Market>('markets', ref => {
      const key = `name.ar`;
      return ref.orderBy(key).where('reviewResult', '==', true);
    }).snapshotChanges()
      .pipe(
        filter(snapshot => snapshot.length > 0),
        mergeMap(markets => {
          return combineLatest(
            of(markets),
            combineLatest(
              markets.map(market => {
                return this.firestore.collection<MarketProduct>(`markets/${market.payload.doc.id}/products`)
                  .snapshotChanges().pipe(filter(snapshot => snapshot.length > 0), map(products => {
                    return products;
                  }))
              })
            )
          )
        }), map(([markets, products]) => {
          let returnedMarkets: MarketId[];
          products.forEach(prodArr => {
            returnedMarkets = markets.map(market => {
              return {
                id: market.payload.doc.id,
                ...market.payload.doc.data() as Market,
                products: prodArr
                  .filter(product => product.payload.doc.ref.path === `markets/${market.payload.doc.id}/products/${product.payload.doc.id}`)
                  .map(product => {
                    return {
                      id: product.payload.doc.id,
                      ...product.payload.doc.data() as MarketProduct
                    } as MarketProductId
                  })
              } as MarketId
            })
          })
          console.log(returnedMarkets);
          return returnedMarkets;
        }))
      .subscribe(markets => {
        this.markets = [];
        this.markets = markets;
        this.newMarketsChanges$.next(this.markets);
      })
  }

如果数据不存在问题，但是如果集合或子集合中没有数据，那么它就会失败，并且不会到达订阅

Answer 1

在处理数据之前，您是否可以先尝试对管道使用过滤器：

...
....
.snapshotChanges()
      .pipe(
            filter(snapshot=> snapshot.exists),
            mergeMap(markets => {.....