使用angularfire2 firestore在Observable中从Observable取值

问题描述

我正在尝试从collection查询document时查询子集合是否存在.我的第一个查询returns/maps数据正确，但是当我尝试查询subcollection时，它将作为observable存储在Observable<data>中.以下是我到目前为止已经/尝试过的内容.

I am trying to query a sub-collection if exists while querying for document from collection. My first query returns/maps data properly but when I try to query the subcollection it will be stored as observable within the Observable<data>. Below is what I have/tried so far.

component.ts

availableCategoriesCollection: AngularFirestoreCollection<Category>;
availableCategories$: Observable<CategoryId[]>;
lstCategories: Observable<any>;

this.availableCategoriesCollection = this.httpDataService.getAllCategories();
this.availableCategories$ = this.availableCategoriesCollection.snapshotChanges().map(data => {
  return data.map(record => {
    const rec = record.payload.doc.data() as Category;
    const cId = record.payload.doc.id;
    return {cId, ...rec};
  });
});

this.lstCategories = this.availableCategories$.map(data => {
  return data.map((rec: CategoryId) => {
    //if a category has subcategory then query the document again for subcollection
    if (rec.hasSubCat) {
      return this.httpDataService.getSubCategory(rec.cId).snapshotChanges().concatMap(d => {
        return d.map(r => {
          const arr: any = {};
          arr.id = r.payload.doc.id;
          arr.itemName = (r.payload.doc.data() as Category).categoryName;
          arr.category = rec.categoryName;
          return arr;
        });
      });
    }else {
      const arr: any = {};
      arr.id = rec.id;
      arr.itemName = rec.categoryName;
      arr.category = 'All';
      return arr;
    }
  });
});

当我查看lstCategories值时，具有subcollection的文档将以Observable的形式返回，而没有subcollection的文档将以id，data >和category.如下所示:

When I look into the lstCategories value, the documents that have subcollection will be returned as Observable and the ones which don't have subcollection returns proper data with id,itemName and category. Something like below:

如何正确订阅sub-query?我在这里想念什么?

How can I properly subscribe to the sub-query? What am I missing here?

推荐答案

所以您的问题是map s回调有时返回一个普通对象，有时返回一个Observable对象.我猜您仍然想发出一个对象数组(包括内部Observable中的对象)，而不是一个接一个地发射项目.

So your problem is that the maps callback sometimes returns a plain object and sometimes returns an Observable. I guess you still want to emit an array of objects (including those inside the inner Observables) instead of emitting items one by one.

我认为最简单的方法是使用forkJoin并始终返回一个Observable(即使结果可能是普通对象):

I think the easiest way to do this is using forkJoin and always returning an Observable (even when the result could be a plain object):

this.lstCategories = this.availableCategories$.mergeMap(data => {
  const observables = data.map((rec: CategoryId) => {
    if (rec.hasSubCat) {
      return this.httpDataService... // Observable
    } else {
      const arr: any = {};
      ...
      return Observable.of(arr); // Observable
    }
  }

  return Observable.forkJoin(observables);
});

还要注意，我必须使用this.availableCategories$.mergeMap()，因为mergeMap将订阅forkJoin Observable并发出其结果.

Also notice that I had to use this.availableCategories$.mergeMap() because mergeMap will subscribe to the forkJoin Observable and emit its result.

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