下面是使用Android Room创建关系在单独的表中的代码示例。
首先,创建一个实体类来表示主表数据:
@Entity(tableName = "users")
public class User {
@PrimaryKey
@ColumnInfo(name = "user_id")
public int userId;
@ColumnInfo(name = "user_name")
public String userName;
}
然后,创建一个实体类来表示关系表数据:
@Entity(tableName = "user_books",
foreignKeys = @ForeignKey(entity = User.class,
parentColumns = "user_id",
childColumns = "user_id",
onDelete = ForeignKey.CASCADE))
public class UserBook {
@PrimaryKey
@ColumnInfo(name = "book_id")
public int bookId;
@ColumnInfo(name = "user_id")
public int userId;
@ColumnInfo(name = "book_name")
public String bookName;
}
在这个示例中,user_books
表表示用户和他们的书籍之间的关系。user_id
列是外键,引用了users
表的user_id
列。
接下来,创建一个DAO接口来定义访问数据库的方法:
@Dao
public interface UserDao {
@Query("SELECT * FROM users")
List getUsers();
@Insert
void insertUser(User user);
}
@Dao
public interface UserBookDao {
@Query("SELECT * FROM user_books")
List getUserBooks();
@Insert
void insertUserBook(UserBook userBook);
}
最后,在Room数据库中定义两个表的访问接口:
@Database(entities = {User.class, UserBook.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
public abstract UserDao userDao();
public abstract UserBookDao userBookDao();
}
现在,你可以在应用程序中使用上述代码示例来创建用户和书籍的关系表,并查询它们的数据。
// 初始化数据库
AppDatabase db = Room.databaseBuilder(getApplicationContext(),
AppDatabase.class, "my-database").build();
// 插入用户和书籍数据
User user = new User();
user.userId = 1;
user.userName = "John Doe";
db.userDao().insertUser(user);
UserBook userBook = new UserBook();
userBook.bookId = 1;
userBook.userId = 1;
userBook.bookName = "Android Programming";
db.userBookDao().insertUserBook(userBook);
// 查询用户和书籍数据
List users = db.userDao().getUsers();
List userBooks = db.userBookDao().getUserBooks();
通过上述代码示例,你可以使用Android Room在单独的表中创建关系,并执行数据库操作。