Transaction

Marks a method in a Dao class as a transaction method.

When used on a non-abstract method of an abstract Dao class, the derived implementation of the method will execute the super method in a database transaction. All the parameters and return types are preserved. The transaction will be marked as successful unless an exception is thrown in the method body.

Example:

@Dao
abstract class SongDao {
    @Insert
    abstract fun insert(song: Song)
    @Delete
    abstract fun delete(song: Song)
    @Transaction
    fun insertAndDeleteInTransaction(newSong: Song, oldSong: Song) {
        // Anything inside this method runs in a single transaction.
        insert(newSong)
        delete(oldSong)
    }
}

When used on a Query method that has a SELECT statement, the generated code for the Query will be run in a transaction. There are 2 main cases where you may want to do that:

• If the result of the query is fairly big, it is better to run it inside a transaction to receive a consistent result. Otherwise, if the query result does not fit into a single android.database.CursorWindow, the query result may be corrupted due to changes in the database in between cursor window swaps.

• If the result of the query is a POJO with {@link Relation} fields, these fields are queried separately. To receive consistent results between these queries, you also want to run them in a single transaction.

Example:

data class AlbumWithSongs : Album (
    @Relation(parentColumn = "albumId", entityColumn = "songId")
    val songs: List<Song>
)

@Dao
public interface AlbumDao {
    @Transaction
    @Query("SELECT * FROM album")
    fun loadAll(): List<AlbumWithSongs>
}

If the query is asynchronous (e.g. returns a androidx.lifecycle.LiveData or RxJava Flowable), the transaction is properly handled when the query is run, not when the method is called.

Putting this annotation on an Insert, Update or Delete method has no impact because those methods are always run inside a transaction. Similarly, if a method is annotated with Query but runs an INSERT, UPDATE or DELETE statement, it is automatically wrapped in a transaction and this annotation has no effect. Room will only perform at most one transaction at a time, additional transactions are queued and executed on a first come, first serve order.

Summary

Transaction()