Android：测试室迁移

Question

I have a migration where I am adding a new column The migration is defined as

val MIGRATION_20_21: Migration = object : Migration(20, 21) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("ALTER TABLE tests ADD COLUMN deleted INTEGER NOT NULL DEFAULT 0")

    }
}

The field is dined as

@ColumnInfo(name = "deleted") var deleted: Boolean = false,

And I have a test case for this migration which looks like

@Test
@Throws(IOException::class)
fun migrate20to21() {
    var db = helper.createDatabase(TEST_DB, 20).apply {
        
        addTestsToDatabase(this, true)
       
        close()
    }

    db = helper.runMigrationsAndValidate(TEST_DB, 21, true,
            MIGRATION_20_21)

    var cursor = db.query("SELECT * FROM tests WHERE id = ?", arrayOf("id0"))
    MatcherAssert.assertThat(cursor, Is(notNullValue()))
    MatcherAssert.assertThat(cursor.moveToFirst(), `is`(true))
    MatcherAssert.assertThat(cursor.getColumnIndex("deleted"), `is`(38))
    MatcherAssert.assertThat(cursor.getInt(cursor.getColumnIndex("deleted")), `is`(0))

I am getting the error Migration didn't properly handle: tests I see a difference in expected and found table info

Expected: deleted=Column{name='deleted', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='null'}, local_created_at=Column{name='local_created_at', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=0, defaultValue='null'}

Found: deleted=Column{name='deleted', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='0'}

Why there is a difference in default value and how can it be null when I have defined it as not null.

Answer 1

Why there is a difference in default value and how can it be null when I have defined it as not null

The reason is that "Expected" is derived from the Entity (at compile time) and then compared to the schema of the actual database that is being/has been opened (what has been "Found" ), so you need to change the Entity to also have a default value of 0, using the @ColumnInfo(defaultValue = "0") .

Without then the default value will be null ie no default value specified.

• The @ColumnInfo 's defaultValue was introduced in Room 2.2.0 , until that version the DEFAULT clause was (I believe) ignored and and expected/found conflict wouldn't arise.

So the Entity should contain

@ColumnInfo(defaultValue = "0")
val deleted: Int = 0

• Or instead of Int = 0 , Boolean = false

Amendment re update showing:-

@ColumnInfo(name = "deleted") var deleted: Boolean = false,

then you need to use :-

@ColumnInfo(name = "deleted", defaultValue = "0") var deleted: Boolean = false,

Re the comment

thank you. when I say var deleted: Boolean = false, is it not the same as saying defaultValue?

Correct it isn't the same.

Setting a value for var/val sets the value of the field when the value isn't provided when instantiating the object and effectively assigns the object a default value.

Whilst defaultValue = "0" says that the column definition includes the DEFAULT 0 in the table's column definition.

eg

 `deleted` INTEGER NOT NULL DEFAULT 0

Without you get

 `deleted` INTEGER NOT NULL

This is what Room EXPECTED (as it is without defaultValue = ) in your case according to the Entity your ALTER column definition that was FOUND differs from the expected (no DEFAULT so Room's interpretation is defaultValue = null).

In your case the defaultValue = "0" is important as 0 (false) will be used as the value for the column for rows that existed before the ALTER, whilst null might result in run time issues.

using var deleted: Boolean = false , is also potentially important as it provides the default value if the value is omitted when instantiating the object.

• Perhaps the best/easiest way to get things as Room expects is to make your Entity changes, then compile (eg Ctrl + F9) and to then inspect the generated java (easily visible from Android View in Android Studio) to then find the class that is named as per the @Database class suffixed with _Impl and then find the createAllTables method which includes the EXPECTED table definitions.

• eg

• 

Another Amendment

Regarding the comment

Does the migration uses the json files created by room?

No, UNLESS AutoMigration is used. AutoMigration builds the code for the migration (I believe) at compile time and hence why it needs schemas and requires exportSchema = true and the underlying gradle schema location directive.

Migrations actually take place at run time and do not need (nor even have access to the saved schemas as I believe that they are not included in the APK).

My current version is 24 but this field was added at version 21. SO even though I add default = 0, it only recreates 24.json. This test case for 20 to 21 fails as 21.json do not have default value.

I suspect that this could be due to different versions of Room. Prior to 2.2.0 defaultValue was not an option and thus it was ignored when comparing expected and found. Noting that this comparison is undertaken after migration(s) have been performed (I believe ALL migrations).