How to get list of documents from a collection in Firestore Android

Question

My structure of Firestore database:

|
|=>root_collection
                  |
                  |=>doc1
                         |                  
                         |=>collection
                  |
                  |=>doc2
                         |                  
                         |=>collection
                  |
                  |=>doc3
                         |                  
                         |=>collection

Now I wanna get list of document from root_collection . There would be a list with following data {"doc1", "doc2", "doc3"} . I need it because I want to make a spinner and put these data in the spinner. Then a user would be choose some document and download it.

I try to use the code below:

firestore.collection("root_collection")
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (QueryDocumentSnapshot document : task.getResult()) {
                            Log.d(TAG,document.getId() + " => " + document.getData());
                        }
                    } else {
                        Log.d(TAG, "Error getting documents: ", task.getException());
                    }
                }
            });

But the code works only then I have structure of data without collections in the documents. In other case there aren't any documents in QueryDocumentSnapshot .

Thanks!

Answer 1

To have a list that contains all the name of your documents within the root_collection , please use the following code:

firestore.collection("root_collection").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            List<String> list = new ArrayList<>();
            for (QueryDocumentSnapshot document : task.getResult()) {
                list.add(document.getId());
            }
            Log.d(TAG, list.toString());
        } else {
            Log.d(TAG, "Error getting documents: ", task.getException());
        }
    }
});

The result in your logcat will be:

[doc1, doc2, doc3]

Remember, this code will work, only if you'll have some properties within those documents, otherwise you'll end ut with an empty list.

Answer 2

You can call collection method to get documentin the root_collections then hold documents ID which will be used to get document's collection later.

create root collection object as:

    data class RootCollection(
        @DocumentId val id: String,
        val sampleField: String?
    ) {
       // empty constructor to allow deserialization
       constructor(): this(null, null)
    }

Then get the collection using FirebaseFirestore method collection as follows:

    val querySnapshot = firestore.collection("root_collection")
                .get()
                .await()
    if(!querySnapshot.isEmpty) {
             Result.SUCCESS(querySnapshot.toObjects(RootCollection::class.java))
     } else {
         Result.ERROR(Exception("No data available"))
    }

Then to get collection in the document call

    firestore.collection("root_collection/$documentId/collection") 

Note that I have use kotlin coroutines await method as describe in this link

The Result class am using to save state of returned data and handle errors.

   sealed class Result<out T: Any> {
      data class SUCCESS<out T: Any>(val data: T) : Result<T>()
      data class ERROR(val e: Exception): Result<Nothing>()
      object LOADING : Result<Nothing>()
   }

Answer 3

Well I think you cant do that now because firestore isn't providing these kind of knowing about the child documents. Your Root Collection will just know its documents.

But what you can do is in your Root collection's document get a reference to your collection and then you can get the documents in that collection.