Entity Framework 6.2 - add children with DbSet.Include() but prevent duplicate children on DbSet.AddRange()

问题

EDIT:

I know how to prevent duplicates when copying entities from one DbContext to another, my question is about their children.

---

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

DbContextHDD also contains:

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

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();

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

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.

How can I avoid that?

---

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

Configuration.ProxyCreationEnabled = false;

I have a many to many relationship:

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; }
}

回答1:

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);

来源：https://stackoverflow.com/questions/57872208/entity-framework-6-2-add-children-with-dbset-include-but-prevent-duplicate-c