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

Question

My structure of Firestore database:

|
|=>root_collection
                  |
                  |=>doc1
                         |                  
            


        

Answer 1

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 2

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.