如何填充房间库中实体的非列字段 [英] How to fill non column field of entity in room library

问题描述

我有一个如下的类实体

@Entity
public class Task {    
    private String name; 
    private Integer ParentId;
    private Integer userId;   
    @Ignore
    private int noOfSubTask;
}

在DAO类中，有一个方法getTaskList()

in DAO class there is a method getTaskList()

@Dao
public interface TaskDao extends Dao<Task> {

@Query("SELECT *,(SELECT count(*) FROM Task b  WHERE a._id = b.ParentId ) AS noOfSubTask FROM Task a ")
LiveData<List<Task>> getTaskList();
}

我想填写 noOfSubTask，其编号由查询的(SELECT b(WHERE a._id = b.ParentId)部分)中的(SELECT count(*))给定， 但问题是它不是一列，因此房间库无法在dao实现(自动生成)类中将其映射到getTaskList方法.

I want to fill noOfSubTask with the number given by (SELECT count(*) FROM Task b WHERE a._id = b.ParentId ) portion of query, but problem is it is not a column so room library does not map it getTaskList method in dao implementation (auto-generated) class.

是否有任何方法可以使用房间库的dao类的任何方法来填充实体的非列字段(如本例中的noOfSubTask)?

Is there any way to fill non column field of entity (like noOfSubTask in my case) using any method of dao class of room library?

推荐答案

几天前，我遇到了同样的问题，并对这个线程进行了研究.我正在使用Kotlin data类来创建数据库实体.鉴于Kotlin数据类不能很好地与继承子类配合使用，因此不是一个可行的选择.我的解决方案是在包装类中使用@Embedded关键字，如下所示:

I faced the same problem a few days ago and had a look at this thread. I am using Kotlin data classes for creating database entities. Given that Kotlin data classes do not play well with inheritance subclassing was not a viable option. My solution was to use the @Embedded keyword in a wrapping class, as follows:

data class TaskToDisplay(@Embedded
                         var task: Task,
                         var noOfSubTask: Int = 0)

此解决方案不会在数据库中创建其他列，最重要的是，将所有Task的字段与Room的响应列进行匹配.

This solution does not create an additional column in the database and most importantly matches all Task's fields with Room's response columns.

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