How to add a new Sub- Collection in a Firestore document Android

Question

I wanted to add new sub-Collection to a existing document. Should i create new POJO class to add Sub-Collection or any other way to do it. I wanted to add new Sub-Collection to existing document ID.I am new to android and Firestore. Thanks in advance. this is my Database

i tried this method but stuck couldn't succed

private void setNewCategory(String downloadUrl){
    FirebaseFirestore db = FirebaseFirestore.getInstance();
    DocumentReference newMainCatRef = db
            .collection("HomeFeed")
            .document("5HEkE0ac7sMa7Gjnvf3E")
            .collection("MainCategory")
            .document();
    itemId = newMainCatRef.getId();
    MainCategory category = new MainCategory();

    category.setCategory_id(itemId);
    category.setCategory_name(category_name.getText().toString());
    category.setCategory_url(downloadUrl);
    category.setPriority(priority.getValue());

    newMainCatRef.set(category).addOnSuccessListener(new OnSuccessListener<Void>() {
        @Override
        public void onSuccess(Void aVoid) {
            Log.d(TAG, "onSuccess: Success on Updating the new Field to cat");
            FirebaseFirestore NC = FirebaseFirestore.getInstance();
            CollectionReference NewCategory = NC
                    .collection("Categories")
                    .document("tUdFCajDcQT995jX6G4k")
                    .collection(category_name.getText().toString());
            NewCategory.add()

            category_name.setText("");
            priority.setValue(0);
            category_image.setImageResource(R.drawable.ic_android);
            category_progress.setVisibility(View.INVISIBLE);

        }
    }).addOnFailureListener(new OnFailureListener() {
        @Override
        public void onFailure(@NonNull Exception e) {
            Toast.makeText(mContext, "Failed to add New Catgory Try again!", Toast.LENGTH_SHORT).show();
        }
    });

}

Answer 1

There is no need to make something special. As I see in your code, you're already using the right reference:

CollectionReference NewCategory = NC
                .collection("Categories")
                .document("tUdFCajDcQT995jX6G4k")
                .collection(category_name.getText().toString());

category_name.getText().toString() being the name of the sub-collection but the problem rises in the following line of code:

NewCategory.add()

Where you aren't passing anything as an argument. You should pass a custom object or a Map in order to be able to write something in the database.