如何删除sqlite-net扩展中多对多表的关系?

问题描述

我使用 SQLite-net Extensions 在学生和班级之间建立了多对多的关系:

public class Students
{
    [PrimaryKey, AutoIncrement]
    public int StudentId { get; set; }
    public string Name { get; set; }

    [ManyToMany(typeof(Students_Classes))]
    public List<Classes> Classes { get; set; }
}

public class Classes
{
    [PrimaryKey, AutoIncrement]
    public int ClassId { get; set; }
    public string Name { get; set; }

    [ManyToMany(typeof(Students_Classes))]
    public List<Students> Students { get; set; }
}

public class Students_Classes
{
    [ForeignKey(typeof(Students))]
    public int StudentFId { get; set; }

    [ForeignKey(typeof(Classes))]
    public int ClassFId { get; set; }
}

我以这种方式添加关系:

I add a relationship this way:

dbConnection.InsertOrReplace(new Students_Classes { StudentFId = sId, ClassFId = cId });

但是当我想删除关系时:

But when I want to delete a relationship:

var validRelation = dbConnection.Find<Students_Classes>(x => x.StudentFId = sId && x.ClassFId = cId);
if (validRelation == null)
    return;

dbConnection.Delete(validRelation);

我收到一条错误消息，说无法删除，因为它没有 PK.我可以让一个学生拥有他的所有课程，删除一个课程，然后再将其与他的课程一起保存，但可能会出现性能问题.

I get an error says cannot delete because it has no PK. I can get a Student with all his classes, remove one class then save it again with his classes but there can be performance issues.

如何删除多对多关系中的关系?谢谢.

How to remove a relationship in a many to many relationship? thanks.

推荐答案

很简单，只需在关系类中添加一个主键即可:

Pretty easy, just add a primary key to your relationship class:

public class Students_Classes
{
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }

    [ForeignKey(typeof(Students))]
    public int StudentFId { get; set; }

    [ForeignKey(typeof(Classes))]
    public int ClassFId { get; set; }
}

无论如何使用 SQLite-Net 扩展，您都不需要自己管理关系表，只需从列表中添加或删除子项并在对象上调用 UpdateWithChildren.

Anyway using SQLite-Net Extensions you don't need to manage the relationship table by yourself, just add or remove the children item from the list and call UpdateWithChildren on the object.

查看集成测试项目以了解其工作原理.

Take a look at the Integration Tests project to see how it works.

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