Google Firestore - How to get several documents by multiple ids in one round-trip?

In practise you would use firestore.getAll like this

async getUsers({userIds}) {
    const refs = userIds.map(id => this.firestore.doc(`users/${id}`))
    const users = await this.firestore.getAll(...refs)
    console.log(users.map(doc => doc.data()))
}

or with promise syntax

getUsers({userIds}) {
    const refs = userIds.map(id => this.firestore.doc(`users/${id}`))
    this.firestore.getAll(...refs).then(users => console.log(users.map(doc => doc.data())))
}

They have just announced this functionality, https://firebase.googleblog.com/2019/11/cloud-firestore-now-supports-in-queries.html .

Now you can use queries like, but mind that the input size can't be greater than 10.

userCollection.where('uid', 'in', ["1231","222","2131"])

With Firebase Version 9 (Dec, 2021 Update):

You can get multiple documents by multiple ids in one round-trip using "documentId()" and "in" with "where" clause:

import {
  query,
  collection,
  where,
  documentId,
  getDocs
} from "firebase/firestore";

const q = query(
  collection(db, "products"),
  where(documentId(), "in", 
    [
      "8AVJvG81kDtb9l6BwfCa", 
      "XOHS5e3KY9XOSV7YYMw2", 
      "Y2gkHe86tmR4nC5PTzAx"
    ]
  ),
);

const productsDocsSnap = await getDocs(q);

productsDocsSnap.forEach((doc) => {
  console.log(doc.data()); // "doc1", "doc2" and "doc3"
});

If you are using flutter, you can do the following:

Firestore.instance.collection('your_collection_name')
  .where(FieldPath.documentId, whereIn:["list", "of", "document", "ids"])
  .getDocuments();

This will return a Future containing List<DocumentSnapshot> which you can iterate as you feel fit.

You could use a function like this:

function getById (path, ids) {
  return firestore.getAll(
    [].concat(ids).map(id => firestore.doc(`${path}/${id}`))
  )
}

It can be called with a single ID:

getById('collection', 'some_id')

or an array of IDs:

getById('collection', ['some_id', 'some_other_id'])

No, right now there is no way to batch multiple read requests using the Cloud Firestore SDK and therefore no way to guarantee that you can read all of the data at once.

However as Frank van Puffelen has said in the comments above this does not mean that fetching 3 documents will be 3x as slow as fetching one document. It is best to perform your own measurements before reaching a conclusion here.

Surely the best way to do this is by implementing the actual query of Firestore in a Cloud Function? There would then only be a single round trip call from the client to Firebase, which seems to be what you're asking for.

You really want to be keeping all of your data access logic like this server side anyway.

Internally there will likely be the same number of calls to Firebase itself, but they would all be across Google's super-fast interconnects, rather than the external network, and combined with the pipelining which Frank van Puffelen has explained, you should get excellent performance from this approach.

You can perform an IN query with the document IDs (up to ten):

import {
    query,
    collection,
    where,
    getDocs,
    documentId,
} from 'firebase/firestore';

export async function fetchAccounts(
    ids: string[]
) {
    // use lodash _.chunk, for example
    const result = await Promise.all(
        chunk(ids, 10).map(async (chunkIds) => {
            const accounts = await getDocs(
                query(
                    collection(firestore, 'accounts'),
                    where(documentId(), 'in', chunkIds)
                ));
            return accounts.docs.filter(doc => doc.exists()).map(doc => doc.data());
        })
    );
    return result.flat(1);
}

Here's how you would do something like this in Kotlin with the Android SDK.
May not necessarily be in one round trip, but it does effectively group the result and avoid many nested callbacks.

val userIds = listOf("123", "456")
val userTasks = userIds.map { firestore.document("users/${it!!}").get() }

Tasks.whenAllSuccess<DocumentSnapshot>(userTasks).addOnSuccessListener { documentList ->
    //Do what you need to with the document list
}

Note that fetching specific documents is much better than fetching all documents and filtering the result. This is because Firestore charges you for the query result set.

For some who are stucked in same problem here is a sample code:

List<String> documentsIds = {your document ids};

FirebaseFirestore.getInstance().collection("collection_name")
.whereIn(FieldPath.documentId(), documentsIds).get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                     for (DocumentSnapshot document : Objects.requireNonNull(task.getResult())) {
                        YourClass object = document.toObject(YourClass.class);
                        // add to your custom list
                    }   
                }
                
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                e.printStackTrace();
            }
        });

if you are using the python firebase admin sdk this is how you query for multiple documents using their uids

from firebase_admin import firestore
import firebase_admin
from google.cloud.firestore_v1.field_path import FieldPath

app = firebase_admin.initialize_app(cred)
client = firestore.client(app)

collection_ref = client.collection('collection_name')
query = collection_ref.where(FieldPath.document_id(), 'in', listOfIds)
docs = query.get()

for doc in docs:
   print(doc.id, doc.to_dict())

Instead of importing FieldPath you can also simply use the string __name__. Now your query will be collection_ref.where('__name__', 'in', listOfIds)

I hope this helps you, it works for me.

getCartGoodsData(id) {

    const goodsIDs: string[] = [];

    return new Promise((resolve) => {
      this.fs.firestore.collection(`users/${id}/cart`).get()
        .then(querySnapshot => {
          querySnapshot.forEach(doc => {
            goodsIDs.push(doc.id);
          });

          const getDocs = goodsIDs.map((id: string) => {
            return this.fs.firestore.collection('goods').doc(id).get()
              .then((docData) => {
                return docData.data();
              });
          });

          Promise.all(getDocs).then((goods: Goods[]) => {
            resolve(goods);
          });
        });
    });
  }

For the ones who want to do it using Angular, here is an example:

First some library imports are needed: (must be preinstalled)

import * as firebase from 'firebase/app'
import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/firestore'

Some configuration for the collection:

yourCollection: AngularFirestoreCollection;

constructor(
    private _db : AngularFirestore,
) { 
    // this is your firestore collection
    this.yourCollection = this._db.collection('collectionName');
}

Here is the method to do the query: ('products_id' is an Array of ids)

getProducts(products_ids) {
    var queryId = firebase.firestore.FieldPath.documentId();
    this.yourCollection.ref.where(queryId, 'in', products_ids).get()
        .then(({ docs }) => {
            console.log(docs.map(doc => doc.data()))
        })
}

Yes, it is possible. Sample in .NET SDK for Firestore:

/*List of document references, for example:
    FirestoreDb.Collection(ROOT_LEVEL_COLLECTION).Document(DOCUMENT_ID);*/
    List<DocumentReference> docRefList = YOUR_DOCUMENT_REFERENCE_LIST;
    
    // Required fields of documents, not necessary while fetching entire documents
    FieldMask fieldMask = new FieldMask(FIELD-1, FIELD-2, ...);
    
    // With field mask
    List<DocumentSnapshot> documentSnapshotsMasked = await FirestoreDb.GetAllSnapshotsAsync(docRefList, fieldMask);
    
    // Without field mask
    List<DocumentSnapshot>documentSnapshots = await FirestoreDb.GetAllSnapshotsAsync(docRefList);

Documentation in .NET:

1. Get all snapshots

2. Field mask

This doesn't seem to be possible in Firestore at the moment. I don't understand why Alexander's answer is accepted, the solution he proposes just returns all the documents in the "users" collection.

Depending on what you need to do, you should look into duplicating the relevant data you need to display and only request a full document when needed.

The best you can do is not use Promise.all as your client then must wait for .all the reads before proceeding.

Iterate the reads and let them resolve independently. On the client side, this probably boils down to the UI having several progress loader images resolve to values independently. However, this is better than freezing the whole client until .all the reads resolve.

Therefore, dump all the synchronous results to the view immediately, then let the asynchronous results come in as they resolve, individually. This may seem like petty distinction, but if your client has poor Internet connectivity (like I currently have at this coffee shop), freezing the whole client experience for several seconds will likely result in a 'this app sucks' experience.