实体框架6.2-使用DbSet.Include()添加子项，但防止DbSet.AddRange()上的子项重复

问题描述

我知道在将实体从一个DbContext复制到另一个DbContext时如何防止重复，我的问题是关于其子代的.

我有两个上下文:DbContextHDD和DbContextUSB，它们都包含一个"Math" Course.

I have two contexts: DbContextHDD and DbContextUSB and they both contain a "Math" Course.

DbContextHDD还包含:

• 历史记录" Course.
• 参加"Math" Course的Student"Simon".
• 参加历史记录" Course的Student杰克".

• a "History" Course.
• a Student "Simon" who attends "Math" Course.
• a Student "Jack" who attends "History" Course.

我想将所有Student实体从DbContextHDD复制到DbContextUSB，并且还包括DbContextUSB中不存在的所有Course实体:

I would like to copy all Student entities from DbContextHDD to DbContextUSB and also include any Course entities not already present in DbContextUSB:

var students = DbContextHDD.Students.Include(s => s.Courses).ToList();
DbContextUSB.Students.AddRange(students);
DbContextUSB.SaveChanges();

这还会将数学"和历史记录" Course实体从DbContextHDD复制到DbContextUSB.

This also copies "Math" and "History" Course entities from DbContextHDD to DbContextUSB.

在Students.AddRange之后，我在DbContextUSB.Courses.Local中有两个数学" Course实体，它们都具有相同的CourseId，并且SaveChanges都失败了，并显示SQLiteException: UNIQUE constraint failed.

After Students.AddRange I have two "Math" Course entities in DbContextUSB.Courses.Local, both with the same CourseId and SaveChanges fails with SQLiteException: UNIQUE constraint failed.

我正在使用代码优先"方法.代理创建被禁用:

I am using the Code-First approach. Proxy creation is disabled:

Configuration.ProxyCreationEnabled = false;

我有很多关系:

public class Student
{
    public Student() 
    {
        this.Courses = new HashSet<Course>();
    }

    public int StudentId { get; set; }
    [Required]
    public string StudentName { get; set; }

    public virtual ICollection<Course> Courses { get; set; }
}

public class Course
{
    public Course()
    {
        this.Students = new HashSet<Student>();
    }

    public int CourseId { get; set; }
    public string CourseName { get; set; }

    public virtual ICollection<Student> Students { get; set; }
}

推荐答案

您是否尝试过应用过滤器?

Have you tried to apply a filter?

var targetStudentIds = DbContextUSB.Students.Select(s => s.StudentId).ToArray();
var sourceStudents = DbContextHDD.Students.Include(s => s.Courses).Where(s => !targetStudentIds.Contains(s.StudentId)).ToArray();

DbContextUSB.Students.AddRange(sourceStudents);

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