Room 2.1.0之后升级的defaultValue迁移要求的混乱

Question

在Room 2.1.0中，常见有以下代码

版本2

@Entity(tableName = "password")
public class Password {
    @ColumnInfo(name = "dummy0")
    @NonNull
    public String dummy0;

}

public class Migration_1_2 extends Migration {
    public Migration_1_2() {
        super(1, 2);
    }

    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE password ADD COLUMN dummy0 TEXT NOT NULL DEFAULT ''");
    }
}

---

迁移指南来自

• https://developer.android.com/training/data-storage/room/migration-db-versions.md#handle-default-values-migrations
• https://developer.android.com/jetpack/androidx/releases/room

很混乱。

注意：如果您的数据库架构已经具有默认值，例如通过 ALTER TABLE x ADD COLUMN y INTEGER NOTNULL DEFAULT z 添加的默认值，并且您决定通过 @ColumnInfo 将默认值定义到相同的列，那么您可能需要提供迁移验证未考虑的默认值。有关详细信息，请参阅房间迁移。

---

在升级到2.2.3之前，有2种可能

1. 如果迁移运行，我们有一dummy0列具有默认值。
2. 或者，dummy0如果这是新数据库，我们有一列没有默认值。

当我们升级到 Room 2.1.0 到 Room 2.2.3 时，这两种情况仍然一切正常，无需添加额外的迁移代码来删除并重新创建表。

我们做进一步的测试。

版本3

@Entity(tableName = "password")
public class Password {
    @ColumnInfo(name = "dummy0")
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;
}

public class Migration_2_3 extends Migration {
    public Migration_2_3() {
        super(2, 3);
    }

    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE password ADD COLUMN dummy1 TEXT NOT NULL DEFAULT ''");
    }
}

仍然工作得很好。

版本4

@Entity(tableName = "password")
public class Password {
    @ColumnInfo(name = "dummy0")
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;

    @ColumnInfo(name = "dummy2", defaultValue = "")
    @NonNull
    public String dummy2;
}

public class Migration_3_4 extends Migration {
    public Migration_3_4() {
        super(3, 4);
    }

    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE password ADD COLUMN dummy2 TEXT NOT NULL DEFAULT ''");
    }
}

仍然工作得很好。

---

那么，我很困惑吗？在什么用例下，我们需要实际删除并重新创建表？

Answer 1

我认为问题不在于添加新列，而在于是否将默认值应用/更改/删除到现有列。然后您可能必须重新创建受影响的表。

例如，如果你改变了：-

@ColumnInfo(name = "dummy0")
@NonNull
public String dummy0;

添加默认值

@ColumnInfo(name = "dummy0", defaultValue = "")
@NonNull
public String dummy0;

然后就会出现模式不匹配，因为预期的模式将具有 DEFAULT ''，而找到的模式（原始数据库）没有默认编码。

• 这需要删除并重新创建表，因为您无法通过 ALTER 来更改列的属性。

如果在 2.2.0 之前，您有一个包含默认值的先前非房间生成的架构，并且实体未相应更改，那么您会遇到冲突，因为预期架构没有默认值，而找到的架构包含DEFAULT = ''.

• 这将需要相应地更改实体。

＃例子

假设当前实体是：-

@Entity(tableName = "password")
public class Password {
    @PrimaryKey
    public Long id;
    @ColumnInfo(name = "dummy0")
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;
}

然后创建表的生成代码是：-

_db.execSQL("CREATE TABLE IF NOT EXISTS `password` (`id` INTEGER, `dummy0` TEXT NOT NULL, `dummy1` TEXT NOT NULL, PRIMARY KEY(`id`))");

• 应用程序与上述内容一起运行以创建数据库。

如果现在版本 2 更改为：-

@Entity(tableName = "password")
public class Password {
    @PrimaryKey
    public Long id;
    @ColumnInfo(name = "dummy0", defaultValue = "" /*<<<<<<<<<< ADDED */)
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;
}

然后生成的代码是：-

_db.execSQL("CREATE TABLE IF NOT EXISTS `password` (`id` INTEGER, `dummy0` TEXT NOT NULL DEFAULT '', `dummy1` TEXT NOT NULL, PRIMARY KEY(`id`))");

使用哑/空迁移 (1-2) 运行，然后：-

• 找到的模式（原始数据库）具有：-defaultValue='null'
• 但预期的模式有：-defaultValue=''''

按照 ：-

2020-01-11 19:11:15.300 12539-12539/a.so59691979 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: a.so59691979, PID: 12539
    java.lang.RuntimeException: Unable to start activity ComponentInfo{a.so59691979/a.so59691979.MainActivity}: java.lang.IllegalStateException: Migration didn't properly handle: password(a.so59691979.Password).
     Expected:
    TableInfo{name='password', columns={dummy0=Column{name='dummy0', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=0, defaultValue=''''}, dummy1=Column{name='dummy1', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=0, defaultValue='null'}, id=Column{name='id', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=1, defaultValue='null'}}, foreignKeys=[], indices=[]}
     Found:
    TableInfo{name='password', columns={dummy0=Column{name='dummy0', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=0, defaultValue='null'}, dummy1=Column{name='dummy1', type='TEXT', affinity='2', notNull=true, primaryKeyPosition=0, defaultValue='null'}, id=Column{name='id', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=1, defaultValue='null'}}, foreignKeys=[], indices=[]}
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)

#修复示例

使用迁移：-

Migration M1_2 = new Migration(1,2) {
    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {

        // CREATE SQL Copied from the generated Java PasswordDatabase_Impl (name changed)
        final String SQL_CREATE_NEW_PASSWORDTABLE =
                "CREATE TABLE IF NOT EXISTS `password_new` (`id` INTEGER, `dummy0` TEXT NOT NULL DEFAULT '', `dummy1` TEXT NOT NULL, PRIMARY KEY(`id`))";
        database.execSQL(SQL_CREATE_NEW_PASSWORDTABLE);
        database.execSQL("INSERT INTO `password_new` SELECT * FROM `password`");
        database.execSQL("ALTER TABLE `password` RENAME TO `password_old`");
        database.execSQL("ALTER TABLE `password_new` RENAME TO `password`");
        database.execSQL("DROP TABLE IF EXISTS `password_old`");
    }
}

解决了问题。

＃代码

以下代码用于生成上述内容：-

密码.java

/*
//Original
@Entity(tableName = "password")
public class Password {
    @PrimaryKey
    public Long id;
    @ColumnInfo(name = "dummy0")
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;
}
*/

// New
@Entity(tableName = "password")
public class Password {
    @PrimaryKey
    public Long id;
    @ColumnInfo(name = "dummy0", defaultValue = "" /*<<<<<<<<<< ADDED */)
    @NonNull
    public String dummy0;

    @ColumnInfo(name = "dummy1")
    @NonNull
    public String dummy1;
}

• 最初使用的是原件

密码数据库.java

@Database(version = 2, entities = {Password.class})
public abstract class PasswordDatabase extends RoomDatabase {
}

• 初始版本为1

MainActivity.java

public class MainActivity extends AppCompatActivity {

    PasswordDatabase passwordDatabase;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        passwordDatabase = Room.databaseBuilder(
                this,
                PasswordDatabase.class,
                "passworddb"
        )
                .allowMainThreadQueries()
                .addMigrations(M1_2)
                .build();
        passwordDatabase.getOpenHelper().getWritableDatabase();
    }

    Migration M1_2 = new Migration(1,2) {
        @Override
        public void migrate(@NonNull SupportSQLiteDatabase database) {

            // CREATE SQL Copied from the generated Java PasswordDatabase_Impl (name changed)
            final String SQL_CREATE_NEW_PASSWORDTABLE =
                    "CREATE TABLE IF NOT EXISTS `password_new` (`id` INTEGER, `dummy0` TEXT NOT NULL DEFAULT '', `dummy1` TEXT NOT NULL, PRIMARY KEY(`id`))";
            database.execSQL(SQL_CREATE_NEW_PASSWORDTABLE);
            database.execSQL("INSERT INTO `password_new` SELECT * FROM `password`");
            database.execSQL("ALTER TABLE `password` RENAME TO `password_old`");
            database.execSQL("ALTER TABLE `password_new` RENAME TO `password`");
            database.execSQL("DROP TABLE IF EXISTS `password_old`");
        }
    };
}

• 最初M1_2的主体是空的（以便强制错误）