Android Dénormalisation: Structure de base de données plate


Exemple

La dénormalisation et une structure de base de données plate sont nécessaires pour télécharger efficacement des appels séparés. Avec la structure suivante, il est également possible de maintenir des relations bidirectionnelles. L'inconvénient de cette approche est que vous devez toujours mettre à jour les données à plusieurs endroits.

Par exemple, imaginez une application qui permet à l'utilisateur de stocker des messages pour lui-même (mémos).

Structure de base de données à plat souhaitée:

|--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"

La classe de mémo utilisée

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;
    }
}

Récupérer les mémos d'un utilisateur

//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) { }
    }

Créer un mémo

//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);

Après le push ou la base de données ressemble à ceci:

|--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"