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:

KotlinJava
@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:

KotlinJava
@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();
}