Displaying a list of data from Firebase using a RecyclerView in Android Studio

Question

I have been working for the last week or so to fix this problem, but nothing I have found seems to help me. I am using Firebase to create an Android app with users, and each user has a history of interactions with other users, so I want to display this history in a list on the main page for any given user. This is what my database looks like in JSON format:

"app" {
  "history" {
    "AAA000" {
      "0" {
        "uid": "BBB111"
      }
      "1" {
        "uid": "CCC222"
      }
    }
    "BBB111" {
      "0" {
        "uid": "AAA000"
      }
    }
    "CCC222" {
      "0" {
        "uid": "AAA000"
      }
    }
  }
  "users" {
    "AAA000" {
      "first_name": "John"
      "last_name": "Doe"
      "username": "johndoe"
    }
    "BBB111" {
      "first_name": "Jane"
      "last_name": "Doe"
      "username": "janedoe"
    }
    "CCC222" {
      "first_name": "Alice"
      "last_name": "Doe"
      "username": "alicedoe"
    }
  }
}

So if AAA000 were the logged in user, I would want to display a list where one element contains the information from BBB111 and the second element contains the information from CCC222. However, the way I am trying to do it, nothing shows up in the list. Here's the code for the RecyclerView in my main page activity:

if (FirebaseAuth.getInstance().getCurrentUser() != null) {
  class Dummy {
    private String uid;

    public Dummy(String uid) {
      this.uid = uid;
    }

    public String getUid() {
      return uid;
    }

    public void setUid(String uid) {
      this.uid = uid;
    }
  }

  final Dummy dummy = new Dummy(FirebaseAuth.getInstance().getCurrentUser().getUid());

  RecyclerView recycler = (RecyclerView) findViewById(R.id.list);
  recycler.setHasFixedSize(true);
  recycler.setLayoutManager(new LinearLayoutManager(this));

  FirebaseRecyclerAdapter<Dummy, HistoryHolder> adapter = new FirebaseRecyclerAdapter<Dummy, HistoryHolder>(Dummy.class, R.layout.list_item,
                    HistoryHolder.class, FirebaseDatabase.getInstance().getReference("history").child(dummy.getUid())) {
    public void populateViewHolder(final HistoryHolder user_holder, Dummy user, int position) {
      DatabaseReference user_reference = FirebaseDatabase.getInstance().getReference("users").child(user.getUid());
      user_reference.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
          user_holder.name.setText(dataSnapshot.child("first_name").getValue().toString());
          user_holder.username.setText(dataSnapshot.child("username").getValue().toString());
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
      });
    }
  };
  recycler.setAdapter(adapter);
}

And here's the HistoryHolder:

private class HistoryHolder extends RecyclerView.ViewHolder {
  public TextView name;
  public TextView username;

  public HistoryHolder(View view) {
    super(view);

    name = (TextView) view.findViewById(R.id.item_name);
    username = (TextView) view.findViewById(R.id.item_username);

    view.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        // Do stuff
      }
    });
  }
}

Whenever I run the app, the RecyclerView seems to be completely empty, and I have no idea why. I'm very new at this, so any suggestions (both about how to fix my problem and how to improve my code generally) would be very much appreciated!!!