Create trigger using Room Database(Room Persistenc

问题:

How to "Create Trigger" using Room Persistence library

CREATE TRIGGER  IF NOT EXISTS delete_till_10 INSERT ON user WHEN (select count(*) from user)>9

BEGIN 

    DELETE FROM user WHERE id IN  (SELECT id FROM user ORDER BY id limit (select count(*) -9 from user));

END

回答1:

Call getOpenHelper() on your RoomDatabase. This gives you a SupportSQLiteOpenHelper, which has an API reminiscent of SQLiteOpenHelper. On there, call getWritableDatabase() to get a SupportSQLiteDatabase, and on there use execSQL() to execute your SQL statements. A RoomDatabase.Callback is one place to execute this sort of SQL, as AdamMc331 illustrates in this Kotlin snippet.

IOW, Room does not really help with this scenario, but you can always work with the lower-level database API for cases like this one.