Persistence Layer in Open Event Organizer Android App

Open Event Organizer is an Event Managing Android App with the core features of Attendee Check In by QR Code Scan and Data Sync with the Open Event API Server. As an event can be large, so the app will be dealing with a large amount of a data. Hence to avoid repetitive network requests for fetching the data, the app maintains a local database containing all the required data and the database is synced with the server. Android provides android.database.sqlite package which contains the API needed to use the database on the Android. But it is really not a good practice to use the sqlite queries everywhere in the app. So there comes a persistence layer. A persistence layer works between the database and the business logic. Open Event Organizer uses Raizlabs’s DbFlow, an ORM based Android Database Library for the same. I will be talking about its implementation through the app in this blog.

First of all, you declare the base class of the database which is used to create the database by Android for the app. You declare all the base constants here. The class looks like:

   name = OrgaDatabase.NAME,
   version = OrgaDatabase.VERSION,
public class OrgaDatabase {
   public static final String NAME = "orga_database";
   public static final int VERSION = 2;

Initialise the database in the Application class using FlowManager provided by the library. Choose the Application class to do this to ensure that the library finds the generated code in the DbFlow.

   new FlowConfig.Builder(context)
           new DatabaseConfig.Builder(OrgaDatabase.class)

The database is created now. For tables creation, DbFlow uses model classes which must be annotated using the annotations provided by the library. The basic annotations are – @Table, @PrimaryKey, @Column, @ForeignKey etc.

For example, the Attendee class in the app looks like:

@Table(database = OrgaDatabase.class)
public class Attendee ... {

   public long id;

   public boolean checkedIn;
       onDelete = ForeignKeyAction.CASCADE,
       onUpdate = ForeignKeyAction.CASCADE)
   public Order order;

This will create a table named attendee with the columns and relationships annotated. Now comes the part of accessing data from the database. Open Event App uses RxJava’s support to the DbFlow library which enables async data accessing. The getItems method from DataBaseRepository looks like:

public <T> Observable<T> getItems(Class<T> typeClass, SQLOperator... conditions) {
   return RXSQLite.rx(
       .flattenAsObservable(items -> items);


The method returns an observable emitting the items from the result. For data saving, the method looks like:

DatabaseDefinition database = FlowManager.getDatabase(OrgaDatabase.class);
FastStoreModelTransaction<T> transaction = FastStoreModelTransaction


And for updating data, the method looks like:

ModelAdapter<T> modelAdapter = FlowManager.getModelAdapter(classType);

DbFlow provides DirectModelNotifier which is used to get notified of the database change anywhere in the app. Open Event App uses PublishSubjects to send notifications on database change event. The implementation of the DatabaseChangeListener in the app looks like:

public class DatabaseChangeListener<T> ... {
   private PublishSubject<ModelChange<T>> publishSubject = PublishSubject.create();
   private DirectModelNotifier.ModelChangedListener<T> modelModelChangedListener;
   public void startListening() {
       modelModelChangedListener = new DirectModelNotifier.ModelChangedListener<T>() {
           public void onTableChanged(@Nullable Class<?> aClass, @NonNull BaseModel.Action action) {
               // No action to be taken
           public void onModelChanged(@NonNull T model, @NonNull BaseModel.Action action) {
               publishSubject.onNext(new ModelChange<>(model, action));
       DirectModelNotifier.get().registerForModelChanges(classType, modelModelChangedListener);

The class is used in the app to get notified of the data change and to update the required local data fields using data from item emitted by the publishSubject of the class. This is used in the app where same data is accessed at more than one places. For example, There are two fragments – AttendeesFragment and AttendeeCheckInFragment from which an attendee’s check in status is toggled. So when the status is toggled from AttendeeCheckInFragment, the change must be updated in the AttendeesFragment’s attendees list. This is carried out using DatabaseChangeListener in the AttendeesPresenter which provides attendees list to the AttendeesFragment. And on the change in the attendee’s check in status, AttendeePresenter’s attendeeListener listens for the change and update the attendee in the list accordingly.

1. Raizlabs’s DbFlow , an ORM Android Database Library Github Repo Link
2. DbFlow documentation
3. Android database managing API android.database.sqlite

Doing a table join in Android without using rawQuery

The Open Event Android App, downloads data from the API (about events, sessions speakers etc), and saves them locally in an SQLite database, so that the app can work even without internet connection.

Since there are multiple entities like Sessions, Speakers, Events etc, and each Session has ids of speakers, and id of it’s venue etc, we often need to use JOIN queries to join data from two tables.


Android has some really nice SQLite helper classes and methods. And the ones I like the most are the SQLiteDatabase.query, SQLiteDatabase.update, SQLiteDatabase.insert ones, because they take away quite a bit of pain for typing out SQL commands by hand.

But unfortunately, if you have to use a JOIN, then usually you have to go and use the SQLiteDatabase.rawQuery method and end up having to type your commands by hand.

But but but, if the two tables you are joining do not have any common column names (actually it is good design to have them so – by having all column names prefixed by tablename_ maybe), then you can hack the usual SQLiteDatabase.query() method to get a JOINed query.

Now ideally, to get the Session where speaker_id was 1, a nice looking SQL query should be like this –

SELECT * FROM speaker INNER JOIN session
ON speaker_id = session_speaker_id
WHERE speaker_id = 1

Which, in android, can be done like this –

String rawQuery = "SELECT * FROM " + SpeakerTable.TABLE_NAME + " INNER JOIN " + SessionTable.TABLE_NAME
        + " ON " + SessionTable.EXP_ID + " = " + SpeakerTable.ID
        + " WHERE " + SessionTable.ID + " = " +  id;
Cursor c = db.rawQuery(

But of course, because of SQLite’s backward compatible support of the primitive way of querying, we turn that command into

FROM session, speaker
WHERE speaker_id = session_speaker_id AND speaker_id = 1

Now this we can write by hacking the terminology used by the #query() method –

Cursor c = db.query(
        SessionTable.TABLE_NAME + " , " + SpeakerTable.TABLE_NAME,
        Utils.concat(SessionTable.PROJECTION, SpeakerTable.PROJECTION),
        SessionTable.EXP_ID + " = " + SpeakerTable.ID + " AND " + SpeakerTable.ID + " = " +  id,

To explain a bit, the first argument String tableName can take table1, table2 as well safely, The second argument takes a String array of column names, I concatenated the two projections of the two classes. and finally, put by WHERE clause into the String selection argument.

You can see the code for all database operations in the android app here