Query Firestone Database with Joins

MainActivity.java

package com.rowland.foodorder;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.firestore.CollectionReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.google.firebase.firestore.WriteBatch;

import java.util.HashMap;
import java.util.Map;

public class MainActivity extends AppCompatActivity {

    private static final String LOG_TAG = MainActivity.class.getSimpleName();
    private FirebaseFirestore db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        db = FirebaseFirestore.getInstance();

        WriteBatch batch = db.batch();

        Group group1 = new Group(Group.MERCHANT);
        Group group2 = new Group(Group.CLIENT);

        CollectionReference groupCollectionRef = db.collection("groups");
        batch.set(groupCollectionRef.document(group1.name), group1);
        batch.set(groupCollectionRef.document(group2.name), group2);

        User user1 = new User("rowlandoti", "rowlandotieno@gmail.com", "0710805009");
        User user2 = new User("kevanlan", "kevanlanister@gmail.com", "0716855689");

        CollectionReference userCollectionRef = db.collection("users");
        batch.set(userCollectionRef.document(user1.username), user1);
        batch.set(userCollectionRef.document(user2.username), user2);

        Map<String, Object> merchantUsers = new HashMap<>();
        merchantUsers.put(user1.username, true);
        merchantUsers.put(user2.username, true);
        Map<String, Object> clientUsers = new HashMap<>();
        clientUsers.put(user2.username, true);

        CollectionReference groupUsersCollectionRef = db.collection("groupusers");
        CollectionReference userGroupsCollectionRef = db.collection("usergroups");

        batch.set(groupUsersCollectionRef.document(Group.MERCHANT), merchantUsers);
        batch.set(groupUsersCollectionRef.document(Group.CLIENT), clientUsers);

        Map<String, Object> user1Groups = new HashMap<>();
        user1Groups.put(Group.MERCHANT, true);
        user1Groups.put(Group.CLIENT, true);
        Map<String, Object> user2Groups = new HashMap<>();
        user2Groups.put(Group.CLIENT, true);

        batch.set(userGroupsCollectionRef.document(user1.username), user1Groups);
        batch.set(userGroupsCollectionRef.document(user2.username), user2Groups);

        batch.commit().addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                if (task.isSuccessful()) {
                    Toast.makeText(MainActivity.this, "Is Successful", Toast.LENGTH_SHORT).show();
                }
            }
        });

    }

    @Override
    public void onResume() {
        super.onResume();

        db.collection("groupusers")
                .document(Group.MERCHANT)
                .addSnapshotListener(new EventListener<DocumentSnapshot>() {
                    @Override
                    public void onEvent(@Nullable DocumentSnapshot documentSnapshot, @Nullable FirebaseFirestoreException e) {
                        if (documentSnapshot != null) {
                            Map<String, Object> merchantKeys = documentSnapshot.getData();
                            for (Map.Entry<String, Object> entry : merchantKeys.entrySet()) {
                                Toast.makeText(MainActivity.this, entry.getKey().toString(), Toast.LENGTH_SHORT).show();
                            }
                        }
                    }
                });
    }
}