How do I retrieve array data from firebase database?

Question

I retrieved data from firebase database in recylerview. But I have in post node "postimg" has a another list item.

For this I use one more listener in onBindViewHolder. But this does not seem right to me.

So how to do all this works in mainactivity and pass in adapter?

Here is my code:

@Override
    public void onBindViewHolder(@NonNull final PostHolder postHolder, final int i) {
        postHolder.setData(mPost.get(i));
        final String PostKey=mPost.get(i).getPostid();
        FirebaseAuth mAuth=FirebaseAuth.getInstance();
        final String currentUserID=mAuth.getCurrentUser().getUid();
        final DatabaseReference post=FirebaseDatabase.getInstance().getReference().child("Posts");
post.child(PostKey).child("postimg").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot)
            {
                if (dataSnapshot.exists())
                {
                    for (DataSnapshot dataSnapshot1:dataSnapshot.getChildren())
                    {
                        String postimagelink =dataSnapshot1.getValue().toString();
                        postimagelist.add(postimagelink);
                    }


                    String[] urls =postimagelist.toArray(new String[postimagelist.size()]);
                    postHolder.mPager.setAdapter(new SlidingImage_Adapter(mContext,urls));

                    postHolder.indicator.setViewPager(postHolder.mPager);

                    final float density = mContext.getResources().getDisplayMetrics().density;
                    postHolder.indicator.setRadius(5 * density);

                    postHolder.NUM_PAGES = urls.length;



                    postHolder.indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener()
                    {

                        @Override
                        public void onPageSelected(int position) {
                            postHolder.currentPage = position;
                        }

                        @Override
                        public void onPageScrolled(int pos, float arg1, int arg2) {
                        }

                        @Override
                        public void onPageScrollStateChanged(int pos) {
                        }
                    });
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

this my query in MainActivity.

mAdapter = new PostAdapter(MainActivity.this);
query = PostRef
                    .orderByChild("timestamp")
                    .limitToLast(mPostsPerPage);
query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                List<Post> userModels = new ArrayList<>();


                for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                    String o=userSnapshot.getKey();
                    userModels.add(userSnapshot.getValue(Post.class));
//Here I want to retrieve no of "postimg"child and its value and pass in adapter//

                }

                mAdapter.addAll(userModels);

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });