Cloud FireStore：使用查询检索1个文档

Question

I'm trying to retrieve a single document from a collection. I'm now using the code below that returns a collections of items, but I know that there is only one item. So it ain't that clean.

Setup:

private db: AngularFirestore
private itemSubs: Subscription[] = [];
itemAd= new Subject<Item>();

fetchItemFromDatabase(itemId: string) {
    this.itemSubs.push(
      this.db.collection('items', id => id.where('itemId', '==', itemId)).valueChanges().subscribe((items: Item[]) => {
        this.itemAd.next(items);
      }));
  }

I tried to do it with this.db.collection('items').doc(itemId).get() , but I'm getting an error on get() that it's not found/supported. I also didn't got autocompletion when trying to call this methode (methode found in the official cloud firestore documents).

I looked at around at some other solutions and then tried it with this.db.collection('items').doc(itemId).ref.get().then(...) , but here I got an empty doc back.

So I'm a bit stuck at the moment and I don't want to use that whole collections logic when I know there is only 1 item in it.

Answer 1

There may be multiple documents with itemId equal to a given value. While you may know that there is only one in your app, the database and API cannot know nor enforce that. For that reason the query you run will always return a query snapshot that potentially contains multiple documents.

this.db.collection('items', id => id.where('itemId', '==', itemId))

If you want to enforce that there is only one document with the given item ID, consider using that item ID as the document name instead of storing it as a field in the document.

There can be only one document with a given name, so that means the ID is guaranteed to be unique. And you can then retrieve that document with:

this.db.collection('items').doc(itemId)