2.1.0 及更高版本的 Room 持久性库为 SQLite 数据库视图提供了支持,从而允许您将查询封装到类中。Room 将这些查询支持的类称为视图,在 DAO 中使用时,它们的行为与简单数据对象的行为相同。
创建视图
如需创建视图,请将 @DatabaseView
注释添加到类中。将注释的值设为类应该表示的查询。
以下代码段提供了一个视图示例:
@DatabaseView("SELECT user.id, user.name, user.departmentId," +
"department.name AS departmentName FROM user " +
"INNER JOIN department ON user.departmentId = department.id")
data class UserDetail(
val id: Long,
val name: String?,
val departmentId: Long,
val departmentName: String?
)
@DatabaseView("SELECT user.id, user.name, user.departmentId," +
"department.name AS departmentName FROM user " +
"INNER JOIN department ON user.departmentId = department.id")
public class UserDetail {
public long id;
public String name;
public long departmentId;
public String departmentName;
}
将视图与数据库相关联
如需将此视图添加为应用数据库的一部分,请在应用的 @Database
注释中添加 views
属性:
@Database(entities = [User::class],
views =[UserDetail::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
abstract fun userDao(): UserDao
}
@Database(entities = {User.class}, views = {UserDetail.class},
version = 1)
public abstract class AppDatabase extends RoomDatabase {
public abstract UserDao userDao();
}