使用多对多关系添加对象实体框架

Question

I have a database like lets say a class Student and a class Course. These two have many to many relationship through a join table, holding ids of these two tables which is hidden in EF the way it supposed to.

First if I add two courses to a student. Like John.classes.Add(math); John.classes.Add(physics) John.classes.Add(math); John.classes.Add(physics) where 'John', math and physics are objects of their respective classes. When i save changes, everything happens the ways it should happen. An entry in students table, two entries in courses table and two entries in StudentCourses join table. All good.

But then, when i add another student say 'Bob' with same two classes. Bob.classes.Add(math); Bob.classes.Add(physics); It should add a row in students table adding Bob and two rows in StudentCourses join table. This doesn't happen. A row is added to students table but no rows are being added to StudentCourses table giving error of duplicate entry in courses table. Entity Framework is not adding courses because math and physics already exist in courses table but it should add two entries in StudentCourses join table.

A work around this is by adding an id column in join table and use this table as normal table and manually add entries in StudentCourses table. But i dont want to do this, I want to know the actual solution. Thanks

Answer 1

i solved it by

team ct = context.teams.Find(club.id);

if (ct == null)
{ comp.teams.Add(club); }
else
{ ct.competitions.Add(comp); }

context.SaveChanges();

where comp is competition object. I am sure this is not the actual solution but just a way around. This can't be...