Android Room database transactions

Question

With the new Room Database in Android, I have a requirement where there are two sequential operations that needs to be made:

removeRows(ids);
insertRows(ids);

If I run this, I see (on examining the db) that there are some rows missing - I assume they are being deleted after inserting. viz. the first operation is running in parallel to the second.

If I use a transaction block, such as this, then it's all fine - the first operation seems to complete before doing the second:

roomDb.beginTransaction();
removeRows(ids);
roomDb.endTransaction();

insertRows(ids);

It's also fine if I give a sleep in-between instead:

removeRows(ids);
Thread.sleep(500);

insertRows(ids);

There doesn't seem to be much documentation for Room, and was wondering if I should use the transaction block like the above when I have sequential operations to be done, or is there any better way of doing it.

EDIT : After @CommonsWare pointed out, @Query are asynchronous, while @Insert and @Delete are synchronous. In view of this, how would I get a query which deletes rows to be async:

@Query("DELETE from table WHERE id IN(:ids)")
int removeRows(List<Long> ids);

According to the build output I get Deletion methods must either return void or return int (the number of deleted rows) , if I try to wrap the return type in a Flowable .

Answer 1

As pointed out on documentation for Transaction , you can do following:

 @Dao
 public abstract class ProductDao {
    @Insert
    public abstract void insert(Product product);

    @Delete
    public abstract void delete(Product product);

    @Transaction
    public void insertAndDeleteInTransaction(Product newProduct, Product oldProduct) {
         // Anything inside this method runs in a single transaction.
         insert(newProduct);
         delete(oldProduct);
     }
 }
 

Answer 2

As @CommonsWare pointed out, @Query are asynchronous , while @Insert , @Delete , @Update are synchronous.

If you want to execute multiple queries in single transaction , Room also provides a method for that as mentioned below.

roomDB.runInTransaction(new Runnable() {
        @Override
        public void run() {
            removeRows(ids);
            insertRows(ids);
        }
    });

I hope this will solve your problem.

Answer 3

For Room transactions in Kotlin you can use:

• Interface with implemented method , like:

@Dao 
interface Dao {

    @Insert 
    fun insert(item: Item)

    @Delete 
    fun delete(item: Item)

    @Transaction
    fun replace(oldItem: Item, newItem: Item){
        delete(oldItem)
        insert(newItem)
    }

}

• Or use open function, like:

@Dao 
abstract class Dao {

    @Insert 
    abstract fun insert(item: Item)

    @Delete 
    abstract fun delete(item: Item)

    @Transaction
    open fun replace(oldItem: Item, newItem: Item){
        delete(oldItem)
        insert(newItem)
    }

}

You'll get error: Method annotated with @Transaction must not be private, final, or abstract. without open modifier.

Answer 4

I believe when we are using DAO interfaces, still we can perform transaction using default interface methods. We need to add the annotation @JvmDefault and @Transaction and we can perform any operation inside that, which belong to single transaction.

@Dao
interface TestDao {
    @Insert
    fun insert(dataObj: DataType)

    @Update
    fun update(dataObj: DataType): Completable

    @Delete
    fun delete(dataObj: DataType): Completable

    @Query("DELETE FROM $TABLE_NAME")
    fun deleteAllData()

    @Query("SELECT * FROM $TABLE_NAME ORDER BY id DESC")
    fun getAllData(): Single<List<DataType>>

    @JvmDefault
    @Transaction
    fun singleTransaction(dataList: List<DataType>) {
        deleteAllData()
        dataList.forEach {
            insert(it)
        }
    }
}

Answer 5

here's the solution to this problem:

@Query("SELECT * FROM friend WHERE id = :id")
Friend getFriendByID(int id);
@Delete
void delete(Friend friend);

---

Friend friendToBeDeleted = friendDAO.getFriendByID(id);
friendDAO.delete(friendToBeDeleted);

You have to go through two steps!