Android - Retrieving data from Firebase

Question

I am developing an android application where older kids can pick up younger kids and walk to school. With the application the authenticated (email and password) younger kid can choose between three adresses to get picked up. As of right now my realtime database looks like this:

I want to retrieve the different addresses and the users who picked the addresses. I am thinking I have to use recyclerview to get the data, but I am unsure on if it is possible to do with my database structure.

Answer 1

Using the FirebaseUI database package makes it simple to bind data from the Firebase Realtime Database to your app's UI. Specifically using FirebaseUI with indexed data is applicable for your current database structure.

For example, you'd use something similar to:

// keyQuery - the Firebase location containing the list of keys to be found in dataRef
// dataRef - the Firebase location to watch for data changes. Each key found at 
//           keyRef's location represents a list item.

Query keyQuery = FirebaseDatabase.getInstance().getReference("/Addresses/Street 10/users");
DatabaseReference dataRef = FirebaseDatabase.getInstance().getReference("/User");

FirebaseRecyclerOptions<User> options = new FirebaseRecyclerOptions.Builder<User>()
        .setIndexedQuery(keyQuery, dataRef, User.class)
        .build();

Where your User class is:

public class User {
    private String username;

    public User() {}

    public String getUsername() {
        return this.username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    // ...
}

You can then use the above created options variable to create a FirebaseRecyclerAdapter instance and then call startListening() on it*.

Using FirebaseUI in this way will automatically handle matching the keys under /Addresses/Street 10/users to the /User node.

---

_{* Version 2.x of FirebaseUI uses FirebaseIndexRecyclerAdapter instead and starts listening automatically so doesn't require a startListening() call. The above example is applicable for version 3.0, see FirebaseUI 3.0 upgrade guide.}

Answer 2

Following on from my previous answer, this one should accommodate your requirement to create a list of all addresses and their associated users, which may be closer to what you're looking for.

Again you can use the FirebaseUI database package to simplify the RecyclerView creation.

You'll need to start denormalizing your data, so your data structure should also include usernames in the addresses node:

{
  "Addresses" : {
    "Street 10" : {
      "name" : "Street 10",
      "users" : {
        "VAzdMWafK6cyhmJnOI4br5xiQg93" : "John"
      }
    }
  },
  "User" : {
    "VAzdMWafK6cyhmJnOI4br5xiQg93" : {
      "username" : "John",
      "address" : "Street 10"
    }
  }
}

Note: you only need to add user IDs to their chosen address (and remove the node if they change selection), so don't use "VAzdMWafK6cyhmJnOI4br5xiQg93" : false for addresses the user has not selected as this could cause confusion.

Then you can use:

Query query = FirebaseDatabase.getInstance().getReference("/Addresses");

FirebaseRecyclerOptions<Address> options = new FirebaseRecyclerOptions.Builder<Address>()
        .setQuery(query, Address.class)
        .build();

Where Address is something like:

public class Address {
    private String name;
    private Map<String, String> users;

    public Address() {}

    public Map<String, String> getUsers() {
        return this.users;
    }

    // ...
}

And create a FirebaseRecyclerAdapter instance from the options variable. Then when binding the viewholder in the adapter, you can access the users map to list each user that has selected this address, without the need to load the entire User object unnecessarily.

This pattern is called denormalization and is the suggested approach when using NoSQL databases (like Firebase Realtime Database). The main downside to this is data duplication: so for example, when a user changes their selected address, you'll need to change:

• The address value under the user, and
• the users list under the address.

Likewise, if a user is allowed to change their username, you'll need to update the username under their chosen address as well as in the user's node.

For details on dealing with this, see this answer which explains a number of methods (although the examples are in JavaScript, the premise still applies).