How to retrieve data from Firebase Database with join if key only return true?

问题

I want to ask how to retrieve group that only user2 join in Android?

"users": {
    "user1": {
        "name": "User 1"
    },
    "user2": {
        "name": "User 2"
    }
}

"groups": {
    "groupA": {
        "name": "A Group"
    },
    "groupB": {
        "name": "B Group"
    }
}

"userGroup": {
    "user1": {
        "groupA": true,
        "groupB": true
    },
    "user2": {
        "groupB": true
    }
}

I already try DatabaseReference userGroupRef = rootRef.child("userGroup/user2") but cannot show the name of the groups

But if I use DatabaseReference groupsRef = rootRef.child("groups") the group that user2 not joining also showing.

Does firebase has a query like groupsID in userGroup/user2?

Edit_1

I use FirebaseRecyclerAdapter to show the list from rootRef.child("groups") that shows all the groups even if user2 is not joining.

groupsRef = rootRef.child("groups");

FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Group, GroupHolder>(Group.class, R.layout.group_item_layout, GroupHolder.class, groupsRef) {
    @Override
    protected void populateViewHolder(GroupHolder viewHolder, Group group, int position) {
        viewHolder.setGroupName(group.getName());
    }
};
groupRecyclerView.setAdapter(adapter);

回答1:

This use-case is specifically addressed by the FirebaseIndexRecyclerAdapter. You pass in two references/queries: one for the so-called index (the keys with true values that you want to show) and one for the actual object that you want to load for each matching key.

In your case:

groupsRef = rootRef.child("groups");
indexRef = rootRef.child("userGroup/user2");

FirebaseIndexRecyclerAdapter adapter = new FirebaseIndexRecyclerAdapter <Group, GroupHolder>
  (Group.class, R.layout.group_item_layout, GroupHolder.class, indexRef, groupsRef) {
    @Override
    protected void populateViewHolder(GroupHolder viewHolder, Group group, int position) {
        viewHolder.setGroupName(group.getName());
    }
};

来源：https://stackoverflow.com/questions/42467195/how-to-retrieve-data-from-firebase-database-with-join-if-key-only-return-true