tasks
   - task_key_1:{
       - name: "task 1"
       - ...
       - users{
             user_key_1 : true
         }
   }

users
  - user_key_1{
        tasks{
             task_key_1 : true
        }
  }

myTasks.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {

            taskReference.child(dataSnapshot.getKey()).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                     ...
                    }
                }
                @Override
                public void onCancelled(DatabaseError databaseError) {
                    Log.e(TAG, databaseError.toString());
                }
            });
        }

        public void onChildRemoved(DataSnapshot dataSnapshot) {
            taskReference.child(dataSnapshot.getKey()).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot taskSnapshot) {
                  ......
                }
                @Override
                public void onCancelled(DatabaseError databaseError) {
                    Log.e(TAG, databaseError.toString());
                }
            });

        }

        public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName) {
            Log.w(TAG, "on child changed")
            });
        }

        public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
            Log.w(TAG, "onChildMoved");
        }

        @Override
        public void onCancelled(DatabaseError error) {}

    });