Object-Relational Mapping

DroidParts takes a traditional DAO approach of having simple Entity classes + EntityManagers.


AbstractDBOpenhelper extends SQLiteOpenHelper and replaces it for DroidParts. It contains helpers that create & update tables based on Entity classes. Of course, you still can alter the schema manually.


  1. Subclass AbstractDBOpenhelper.
  2. Return an instance of it in the DependencyProvider (see Dependency Injection).
  3. Override onCreateTables(...) method & call createTables(...) inside it.
  4. See available helpers below.

Helper Methods

  • createTables(SQLiteDatabase db, Class<? extends Entity>... entityClasses)
  • createIndex(SQLiteDatabase db, String table, boolean unique, String firstColumn, String... otherColumns)
  • addMissingColumns(SQLiteDatabase db, Class<? extends Entity>... entityClasses) - adds new columns for model, in onUpgrade(...).
  • dropTables(SQLiteDatabase db, String... optionalTableNames)
  • executeStatements(SQLiteDatabase db, ArrayList<String> statements) - in a transaction.


Model classes must extend Entity or it’s subclass.

Entity already has a public long id field that is mapped to _id column. It will be automatically populated once once an entity is saved or read.

@Table Annotation

  • name - table name.

@Column Annotation

  • name - column name.
  • nullable - null is ok? SQL constraint.
  • unique - should be unique? SQL constraint.
  • eager - for foreign keys, whether should be prefilled at read time.


You’ll typically have an EntityManager subclass for each Entity type and any custom helper methods there. Like so:

public class PostEntityManager extends EntityManager<Post> {

   public PostEntityManager(Context ctx) {
      super(Post.class, ctx);

   public Select<Post> selectPostsAfterYear(int year) {
      return select().where("year", Is.GREATER, year);


CRUD methods

EntityManager comes with a number of data manipulation methods:

  • boolean create(EntityType item), int create(Collection<EntityType> items)
  • EntityType read(long id)
  • boolean update(EntityType item), int update(Collection<EntityType> items)
  • boolean delete(long id), int delete(Collection<EntityType> items)
  • boolean createOrUpdate(EntityType item)

Select, Update, Delete Builders

There are select(), update(), delete() builders to query or manipulate miltiple items.

 // Select is used to provide data to EntityCursorAdapter
Select<EntityType> select = select().columns("_id", "name").where("external_id", Is.EQUAL, 10);

// alternatively, call execute() to get the underlying Cursor
Cursor cursor = select().where("name", Is.LIKE, "%%alex%%").execute();

// use Where object for complex queries
Where haveCoordinaltes = new Where("latitude", Is.NOT_EQUAL, 0).or("longitude", Is.NOT_EQUAL, 0);
select().where("country", Is.EQUAL, "us").where(haveCoordinates);

Misc. methods

Methods that operate on a Select object:

  • long[] readIds(Select<EntityType> select)
  • EntityType readFirst(Select<EntityType> select)
  • ArrayList<EntityType> readAll(Select<EntityType> select)

Methods that operate on an EntityType object:

  • void fillForeignKeys(EntityType item, String... columnNames)


A junction table is required for m2m:

public class TrackToTag extends Entity {
    @Column(nullable = false)
    public Track track;
    @Column(nullable = false)
    public Tag tag;

If column name is not specified via annotation, it will be field_name + _id, e.g. track_id.