Room Data Access Objects (DAO)

DAO

A Data Access Object (DAO) is an abstract class or interface that includes methods to define database queries.

DAOs provide the following advantages:

• No need to use queries
• Easy to mock
• Verified and Parsed at compile time
• Reduced boilerplate

DAO Annotations

Types:

• @Dao
• @Query
• @Insert
• @Delete
• @Update

Insert

@Dao // Required annotation for DAO to be recoginized by Room
public interface MyDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    public void insertUsers(User... users);

    @Insert
    public void insertBothUsers(User user1, User user2);

    @Insert
    public void insertUsersAndFriends(User user, List<User> friends);
}

If the @Insert method receives only 1 parameter, it can return a long, which is the new rowId for the inserted item. If the parameter is an array or a collection, it should return long[] or List<Long> instead.

Update

@Dao
public interface MyDao {
    @Update
    public void updateUsers(User... users);
}

Delete

@Dao
public interface MyDao {
    @Delete
    public void deleteUsers(User... users);
}

Query

Performs read/write operations on the database. Verified at compile time.

Simple Query

@Dao
public interface MyDao {
    @Query("SELECT * FROM user")
    public User[] loadAllUsers();
}

Passing parameters

you can include parameters passed into the method annotated with @Query, by appending a colon (:) to the parameter in the query string.

@Dao
public interface MyDao {
    @Query("SELECT * FROM user WHERE age > :minAge")
    public User[] loadAllUsersOlderThan(int minAge);
}

@Dao
public interface MyDao {
    @Query("SELECT * FROM user WHERE age BETWEEN :minAge AND :maxAge")
    public User[] loadAllUsersBetweenAges(int minAge, int maxAge);

    @Query("SELECT * FROM user WHERE first_name LIKE :search "
           + "OR last_name LIKE :search")
    public List<User> findUserWithName(String search);
}

subsets

public class NameTuple {
    @ColumnInfo(name="first_name")
    public String firstName;

    @ColumnInfo(name="last_name")
    public String lastName;
}

@Dao
public interface MyDao {
    @Query("SELECT first_name, last_name FROM user")
    public List<NameTuple> loadFullName();
}

Can be embedded instead.

Observable queries

return livedata to observe database.

@Dao
public interface MyDao {
    @Query("SELECT first_name, last_name FROM user WHERE region IN (:regions)")
    public LiveData<List<User>> loadUsersFromRegionsSync(List<String> regions);
}

RxJava 2.0 integration

@Dao
public interface MyDao {
    @Query("SELECT * from user where id = :id LIMIT 1")
    public Flowable<User> loadUserById(int id);
}

Querying multiple tables Room will watch all tables for invalidation if they are flowable or livedata and can perform join tables.

@Dao
public interface MyDao {
    @Query("SELECT * FROM book "
           + "INNER JOIN loan ON loan.book_id = book.id "
           + "INNER JOIN user ON user.id = loan.user_id "
           + "WHERE user.name LIKE :userName")
   public List<Book> findBooksBorrowedByNameSync(String userName);
}

Using a POJO to store a join table object

@Dao
public interface MyDao {
   @Query("SELECT user.name AS userName, pet.name AS petName "
          + "FROM user, pet "
          + "WHERE user.id = pet.user_id")
   public LiveData<List<UserPet>> loadUserAndPetNames();

   // You can also define this class in a separate file, as long as you add the
   // "public" access modifier.
   static class UserPet {
       public String userName;
       public String petName;
   }
}

Kotlin RxJava

@Dao
interface UserDao {

    /**
     * Get a user by id.

     * @return the user from the table with a specific id.
     */
    @Query("SELECT * FROM Users WHERE userid = :id")
    fun getUserById(id: String): Flowable<User>

    /**
     * Insert a user in the database. If the user already exists, replace it.

     * @param user the user to be inserted.
     */
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertUser(user: User)

    /**
     * Delete all users.
     */
    @Query("DELETE FROM Users")
    fun deleteAllUsers()
}