Android Firebase Realtime DataBase Denormalization: Flat Database Structure
Example
Denormalization and a flat database structure is neccessary to efficiently download separate calls. With the following structure, it is also possible to maintain two-way relationships. The disadvantage of this approach is, that you always need to update the data in multiple places.
For an example, imagine an app which allows the user to store messages to himself (memos).
Desired flat database structure:
|--database
|-- memos
|-- memokey1
|-- title: "Title"
|-- content: "Message"
|-- memokey2
|-- title: "Important Title"
|-- content: "Important Message"
|-- users
|-- userKey1
|-- name: "John Doe"
|-- memos
|-- memokey1 : true //The values here don't matter, we only need the keys.
|-- memokey2 : true
|-- userKey2
|-- name: "Max Doe"
The used memo class
public class Memo {
private String title, content;
//getters and setters ...
//toMap() is necessary for the push process
private Map<String, Object> toMap() {
HashMap<String, Object> result = new HashMap<>();
result.put("title", title);
result.put("content", content);
return result;
}
}
Retrieving the memos of a user
//We need to store the keys and the memos seperately
private ArrayList<String> mKeys = new ArrayList<>();
private ArrayList<Memo> mMemos = new ArrayList<>();
//The user needs to be logged in to retrieve the uid
String currentUserId = FirebaseAuth.getInstance().getCurrentUser().getUid();
//This is the reference to the list of memos a user has
DatabaseReference currentUserMemoReference = FirebaseDatabase.getInstance().getReference()
.child("users").child(currentUserId).child("memos");
//This is a reference to the list of all memos
DatabaseReference memoReference = FirebaseDatabase.getInstance().getReference()
.child("memos");
//We start to listen to the users memos,
//this will also retrieve the memos initially
currentUserMemoReference.addChildEventListener(new ChildEventListener() {
@Override
public void onChildAdded(DataSnapshot dataSnapshot, String s) {
//Here we retrieve the key of the memo the user has.
String key = dataSnapshot.getKey(); //for example memokey1
//For later manipulations of the lists, we need to store the key in a list
mKeys.add(key);
//Now that we know which message belongs to the user,
//we request it from our memos:
memoReference.child(key).addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
//Here we retrieve our memo:
Memo memo = dataSnapshot.getValue(Memo.class);
mMemos.add(memo);
}
@Override
public void onCancelled(DatabaseError databaseError) { }
});
}
@Override
public void onChildChanged(DataSnapshot dataSnapshot, String s) { }
@Override
public void onChildRemoved(DataSnapshot dataSnapshot) { }
@Override
public void onChildMoved(DataSnapshot dataSnapshot, String s) { }
@Override
public void onCancelled(DatabaseError databaseError) { }
}
Creating a memo
//The user needs to be logged in to retrieve the uid
String currentUserUid = FirebaseAuth.getInstance().getCurrentUser().getUid();
//This is the path to the list of memos a user has
String userMemoPath = "users/" + currentUserUid + "/memos/";
//This is the path to the list of all memos
String memoPath = "memos/";
//We need to retrieve an unused key from the memos reference
DatabaseReference memoReference = FirebaseDatabase.getInstance().getReference().child("memos");
String key = memoReference.push().getKey();
Memo newMemo = new Memo("Important numbers", "1337, 42, 3.14159265359");
Map<String, Object> childUpdates = new HashMap<>();
//The second parameter **here** (the value) does not matter, it's just that the key exists
childUpdates.put(userMemoPath + key, true);
childUpdates.put(memoPath + key, newMemo.toMap());
FirebaseDatabase.getInstance().getReference().updateChildren(childUpdates);
After the push, or database looks like this:
|--database
|-- memos
|-- memokey1
|-- title: "Title"
|-- content: "Message"
|-- memokey2
|-- title: "Important Title"
|-- content: "Important Message"
|-- generatedMemokey3
|-- title: "Important numbers"
|-- content: "1337, 42, 3.14159265359"
|-- users
|-- userKey1
|-- name: "John Doe"
|-- memos
|-- memokey1 : true //The values here don't matter, we only need the keys.
|-- memokey2 : true
|-- generatedMemokey3 : true
|-- userKey2
|-- name: "Max Doe"
