如何从 firebase firestore 异步获取所有父 collections 的所有文档？

Question

I'm trying to get all the documents from all the collections from a firestore database which looks something like this:

-users(collection)
    -user1 (document)
        -snippets(collection)
            -snippetdId1 (document)
            -snippetdId2 (document) 
    -user2 (document)
        -snippets (collection)
            -snippetdId1 (document)
            -snippetdId2 (document)
            -snippetdId3 (document)

let allSnippets = [];
    firebase
      .firestore()
      .collection("users")
      .get()
      .then((snapshot) => {
        snapshot.docs.forEach(async (element) => {
          const snippets = await firebase
            .firestore()
            .collection("users")
            .doc(element.id)
            .collection("snippets")
            .get();
          snippets.forEach(
            async (snippet) => await allSnippets.push(snippet.data())
          );
        });
        console.log(allSnippets); // [](length 0, but has all the elements inside)
      });

The problem I'm having is that the 'allSnippets' array has a length of 0 but it has all the elements that I want, I know the problem is with synchronicity but I don't know how to solve it. I've tried using.then instead but is the same thing, length 0 but has all the elements inside.

Answer 1

The problem is that the console.log(allSnippets) runs before any of the allSnippets.push(snippet.data() has run. If you add some more logging, or run the code in a debugger, you can easily see this.

Normally, you'd use Promise.all to wait until all the get() calls for the snippets are done. But here there's actually an easier way. Since you want to get all snippets from all users, you can use a collection group query . With that, the entire code is reduced to:

firebase.firestore()
    .collectionGroup("snippets")
    .get()
    .then((snippetsSnapshot) => {
      const snippets = snippetsSnapshot.docs.map(doc => doc.data());
      console.log(snippets);
    })