在Android Room中,可以通过使用中间表(junction table)来实现多对多关系。中间表用于连接两个表之间的关系。
以下是一个示例,假设有三个实体类:User、Role和UserRoles中间表。
首先,定义User实体类:
@Entity(tableName = "users")
public class User {
@PrimaryKey
public int userId;
public String username;
}
然后,定义Role实体类:
@Entity(tableName = "roles")
public class Role {
@PrimaryKey
public int roleId;
public String roleName;
}
接下来,定义UserRoles中间表:
@Entity(primaryKeys = {"userId", "roleId"},
foreignKeys = {
@ForeignKey(entity = User.class,
parentColumns = "userId",
childColumns = "userId",
onDelete = ForeignKey.CASCADE),
@ForeignKey(entity = Role.class,
parentColumns = "roleId",
childColumns = "roleId",
onDelete = ForeignKey.CASCADE)
})
public class UserRoles {
public int userId;
public int roleId;
}
然后,在数据库类中创建实体之间的关系:
@Database(entities = {User.class, Role.class, UserRoles.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
public abstract UserDao userDao();
public abstract RoleDao roleDao();
public abstract UserRolesDao userRolesDao();
private static volatile AppDatabase instance;
public static synchronized AppDatabase getInstance(Context context) {
if (instance == null) {
instance = Room.databaseBuilder(context.getApplicationContext(),
AppDatabase.class, "app_database")
.build();
}
return instance;
}
}
最后,在DAO(数据访问对象)中定义查询方法:
@Dao
public interface UserDao {
@Query("SELECT * FROM users")
List getAllUsersWithRoles();
}
这样,你就可以通过UserDao的getAllUsersWithRoles方法获取包含多个表的关系。
请注意,上述代码仅为示例,实际应用中可能需要根据具体情况进行调整。