What's the best way to check if a Firestore record exists if its path is known?

Taking a look at this question it looks like .exists can still be used just like with the standard Firebase database. Additionally, you can find some more people talking about this issue on github here

The documentation states

NEW EXAMPLE

var docRef = db.collection("cities").doc("SF");

docRef.get().then((doc) => {
    if (doc.exists) {
        console.log("Document data:", doc.data());
    } else {
        // doc.data() will be undefined in this case
        console.log("No such document!");
    }
}).catch((error) => {
    console.log("Error getting document:", error);
});

OLD EXAMPLE

const cityRef = db.collection('cities').doc('SF');
const doc = await cityRef.get();
    
if (!doc.exists) {
    console.log('No such document!');
} else {
    console.log('Document data:', doc.data());
}

Note: If there is no document at the location referenced by docRef, the resulting document will be empty and calling exists on it will return false.

OLD EXAMPLE 2

var cityRef = db.collection('cities').doc('SF');

var getDoc = cityRef.get()
    .then(doc => {
        if (!doc.exists) {
            console.log('No such document!');
        } else {
            console.log('Document data:', doc.data());
        }
    })
    .catch(err => {
        console.log('Error getting document', err);
    });

If the model contains too much fields, would be a better idea to apply a field mask on the CollectionReference::get() result (let's save more google cloud traffic plan, \o/). So would be a good idea choose to use the CollectionReference::select() + CollectionReference::where() to select only what we want to get from the firestore.

Supposing we have the same collection schema as firestore cities example, but with an id field in our doc with the same value of the doc::id. Then you can do:

var docRef = db.collection("cities").select("id").where("id", "==", "SF");

docRef.get().then(function(doc) {
    if (!doc.empty) {
        console.log("Document data:", doc[0].data());
    } else {
        console.log("No such document!");
    }
}).catch(function(error) {
    console.log("Error getting document:", error);
});

Now we download just the city::id instead of download entire doc just to check if it exists.

I Encountered Same Problem recently while using Firebase Firestore and i used following approach to overcome it.

mDb.collection("Users").document(mAuth.getUid()).collection("tasks").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if (task.isSuccessful()) {
                if (task.getResult().isEmpty()){
                    Log.d("Test","Empty Data");
                }else{
                 //Documents Found . add your Business logic here
                }
            }
        }
    });

task.getResult().isEmpty() provides solution that if documents against our query was found or not

Depending on which library you are using, it may be an observable instead of a promise. Only a promise will have the 'then' statement. You can use the 'doc' method instead of the collection.doc method, or toPromise() etc. Here is an example with the doc method:

let userRef = this.afs.firestore.doc(`users/${uid}`)
.get()
.then((doc) => {
  if (!doc.exists) {

  } else {

  }
});

})

Hope this helps...

If for whatever reason you wanted to use an observable and rxjs in angular instead of a promise:

this.afs.doc('cities', "SF")
.valueChanges()
.pipe(
  take(1),
  tap((doc: any) => {
  if (doc) {
    console.log("exists");
    return;
  }
  console.log("nope")
}));