How to perform join query in Firebase?

Question

After retrieving group key in HashMap how to perform join query which shows only those grp details which have member as that particular user. And if this structure is wrong please help me with this.

Structure:

Answer 1

Use DatabaseReference inside another DatabaseReference:

 // any way you managed to go the node that has the 'grp_key'
    DatabaseReference MembersRef = FirebaseDatabase.getInstance()
            .getReference()
            .child("Members")
            .child("1CkPG20Tt2dzrVkYkdfCLo")
            .orderByKey().equalTo("-KYnhiAucnasdkadNC")
            .addValueEventListener(
                    new ValueEventListener()
                    {
                        @Override
                        public void onDataChange(DataSnapshot dataSnapshot)
                        {
                            for (DataSnapshot child : dataSnapshot.getChildren())
                            {

                                Map<String, Object> valuesMap = (HashMap<String, Object>) dataSnapshot.getValue();

                                // Get push id value.
                                String key = valuesMap.get("grp_key");


                                // HERE WHAT CORRESPONDS TO JOIN
                                DatabaseReference chatGroupRef = FirebaseDatabase.getInstance().getReference()
                                        .child("Chat_groups")
                                        .orderByKey().equalTo(key)
                                        .addValueEventListener(
                                                new ValueEventListener()
                                                {
                                                    @Override
                                                    public void onDataChange(DataSnapshot dataSnapshot)
                                                    {
                                                        // repeat!!
                                                    }

                                                    @Override
                                                    public void onCancelled(DatabaseError databaseError)
                                                    {

                                                    }
                                                }
                                        )
                            }
                        }

                        @Override
                        public void onCancelled(DatabaseError databaseError)
                        {

                        }
                    }
            );