How to get an Array of Objects from Firestore

Question

What would be the right way to query a field that is an array of maps.

Currently the structure is

Collection1
     Document1
         -papers:                <---- This is an array  
              (0): 
                 -Name:abc
                 -Id:123 
              (1): 
                 -Name:xyz
                 -Id:456

And this is my code

DocumentReference docRef = db.collection("Collection1").document("Document1");
        docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                if (task.isSuccessful()) {
                    DocumentSnapshot document = task.getResult();
                    if (document != null && document.exists()) {
                       //?? how can I retrieve papers
                }
            }
        });

Basically do I retrieve it and cast it as an ArrayList> and then loop through it to create my final ArrayList ?

Or how does it work ?

Answer 1

As per official documentation regarding arrays:

Although Cloud Firestore can store arrays, it does not support querying array members or updating single array elements.

If you only want to get the entire papers array you need to iterate over a Map like this:

Map<String, Object> map = document.getData();
for (Map.Entry<String, Object> entry : map.entrySet()) {
    if (entry.getKey().equals("papers")) {
        Log.d("TAG", entry.getValue().toString());
    }
}

But note, even if papers object is stored in the database as an array, entry.getValue() returns an ArrayList, not an array.

Edit 13 Aug 2018:

According to the updated documentation regarding array membership, now it is possible to filter data based on array values using whereArrayContains() method. A simple example would be:

CollectionReference citiesRef = db.collection("cities");
citiesRef.whereArrayContains("regions", "west_coast");

This query returns every city document where the regions field is an array that contains west_coast. If the array has multiple instances of the value you query on, the document is included in the results only once.