如何将数据添加到新创建的 Room 数据库？

Question

I've tried to import data in the onCreate method:

    db = Room.databaseBuilder(applicationContext,
            AppDatabase::class.java,
            "my-database")
        .allowMainThreadQueries()
        //.fallbackToDestructiveMigration()
        .addCallback(object: RoomDatabase.Callback() {
            override fun onCreate(db: SupportSQLiteDatabase) {
                Log.d("RoomDatabase.Callback", "onCreate called")
                importData()
            }
        })
        .build()

but that leads to a

IllegalStateException: getDatabase called recursively

Doing the import after the database creation also doesn't work:

    var performInitialImport = false

    db = Room.databaseBuilder(applicationContext,
            AppDatabase::class.java,
            "my-database")
        .allowMainThreadQueries()
        //.fallbackToDestructiveMigration()
        .addCallback(object: RoomDatabase.Callback() {
            override fun onCreate(db: SupportSQLiteDatabase) {
                Log.d("RoomDatabase.Callback", "onCreate called")
                performInitialImport = true
            }
        })
        .build()

    Log.d("PerformInitialImport", "$performInitialImport")

    if (performInitialImport) {
        importData()
    }

because the Callback isn't executed before Control Flow continues after the database was built. This is evident by the fact that

D/PerformInitialImport: false

comes before

D/RoomDatabase.Callback: onCreate called

in the App's Run log.

I'm lost. How am I meant to import data into the database upon first creation?

Update: The importData method uses the AppDatabase instance created on the first line. The db parameter passed to onCreate is the underlying low level database. Naturally, I want to use the DAOs and the Entities I already have to fill the database...

Answer 1

The reason why you are getting the behaviour is that the build does not open the database and it's not until you use the database that it is opened and then if the database doesn't exist it will then be created and onCreate is called.

As such performInitialImport will always be false unless you access the database, forcing it to be opened.

You need to open (access) the database before doing the test.

Example:-

class MainActivity : AppCompatActivity() {

    var firstrun = false
    var myTableDao :MyTableDao? = null


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        Log.d("ROOMBUILD","Building Room Database")
        val db = Room.databaseBuilder(this,AppDatabase::class.java,"mydb")
            .allowMainThreadQueries()
            .addCallback(object: RoomDatabase.Callback()
            {
                override fun onCreate(db: SupportSQLiteDatabase) {
                    Log.d("ONCREATE","onCreate called.")
                    super.onCreate(db)
                    firstrun = true;
                }
            }
            )
            .build()
        Log.d("ROOMBUILD","Room database built")
        myTableDao = db!!.myTableDao()
        //myTableDao!!.getAll() //would also work assuming getAll is defined in the Dao
        db.openHelper.writableDatabase //<<<<<<<<<< Force open
        if (firstrun) {
            importData()
        }
         for (m in myTableDao!!.getAll()) {
             Log.d("MYTABLEINFO","Names is " + m.name + " ID is " + m.id)
         }
    }

    fun importData() {
        Log.d("IMPORTING","Data being imported")
        var mytable = MyTable()
        mytable.name = "MyData"
        myTableDao?.insert(mytable)
    }
}

Results

When installed:-

 2019-11-08 10:36:18.629 D/ROOMBUILD: Building Room Database 2019-11-08 10:36:18.642 D/ROOMBUILD: Room database built 2019-11-08 10:36:18.664 D/ONCREATE: onCreate called. 2019-11-08 10:36:18.669 D/IMPORTING: Data being imported 2019-11-08 10:36:18.675 D/MYTABLEINFO: Names is MyData ID is 1

Subsequent runs:-

 2019-11-08 10:37:07.759 D/ROOMBUILD: Building Room Database 2019-11-08 10:37:07.772 D/ROOMBUILD: Room database built 2019-11-08 10:37:07.789 D/MYTABLEINFO: Names is MyData ID is 1