创建视图并将其关联到数据库

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();
}