How to use Entity Framework 6 to update many-to-many relationship? - entity-framework

Question

Someone else asked a similar question here: How can I use EF6 to update a many to many table
I mention that up front because I couldn't get any of the solutions given to work.
I also studied the solution give on Code Project: http://www.codeproject.com/Tips/893609/CRUD-Many-to-Many-Entity-Framework, but this doesn't work for me either.
I'm trying to keep this as simple as possible.
I have two tables: dbo.Teacher and dbo.Student. Each has an "ID" column that servers as a primary key. I also have a third table called dbo.StudentTeacher which has exactly two columns, both are non-nullable and foreign keyed to the previous two tables; in other words, it establishes a many-to-many relationship between teachers and students. As expected, the EDMX designed shows only dbo.Student and dbo.Teacher and infers the relationship between them.
Here is a script for the above; there is nothing else in the database.
CREATE TABLE dbo.Teacher
(
Id INT NOT NULL PRIMARY KEY,
Name VARCHAR(MAX)
);
CREATE TABLE dbo.Student
(
Id INT NOT NULL PRIMARY KEY,
Name VARCHAR(MAX)
);
CREATE TABLE dbo.TeacherStudent
(
TeacherId INT NOT NULL FOREIGN KEY REFERENCES Teacher(Id),
StudentId INT NOT NULL FOREIGN KEY REFERENCES Student(Id)
);
INSERT INTO Teacher(Id, Name)
VALUES
(101, 'Tom');
INSERT INTO Student(Id, Name)
VALUES
(201, 'Sue'),
(202, 'Stan');
INSERT INTO TeacherStudent(TeacherId, StudentId)
VALUES
(101, 201);
Now that I've established my data structures, I want to accomplish a very simple task. From the script above, you can see that we have one teacher named "Tom" who has a student named "Sue". We also have a student named "Stan" with no teacher. My task is to modify the database so that Sue is no longer Tom's student and Stan becomes Tom's student.
To accomplish this, I wrote the following code:
class Program
{
static void Main(string[] args)
{
using (var entities = new TestEntities())
{
// There is only one teacher in the system.
Teacher teacher = entities.Teachers.Single();
// This teacher has a student #201: Sue.
// I want to replace her with student #202: Stan.
teacher.Students.Clear();
teacher.Students.Add(new Student() { Id = 202 });
entities.SaveChanges();
}
}
}
It looks very simple: clear the students associated with Tom and then add Stan as Tom's student. However, when I run the code, I get the following error: Unable to update the EntitySet 'TeacherStudent' because it has a DefiningQuery and no <DeleteFunction> element exists in the <ModificationFunctionMapping> element to support the current operation.
I tried simplifying the problem by trying to just remove Sue from being Tom's student without adding Stan, and I get the exact same error message.
As I understand, this error normally occurs when Entity Framework doesn't have enough information to do what you want it to do, but I really can't see what's missing. There are two simple tables with a join table between them and I need to be able to change which rows are related to which other rows.
I should also note that if I'm not mistaken, the change that I wish to make in this example should affect only the dbo.TeacherStudent table; the other two tables should not be touched.

Answer 1

Okay, after some more Google-Fu, I figured it out.
Even tho the join table must have only two columns with each column foreign keyed to the two tables to be related, the join table still needs a primary key, which can be a composite of the two foreign keys.
Thus, dbo.TeacherStudent should be created with this:
CREATE TABLE dbo.TeacherStudent
(
TeacherId INT NOT NULL FOREIGN KEY REFERENCES Teacher(Id),
StudentId INT NOT NULL FOREIGN KEY REFERENCES Student(Id),
PRIMARY KEY(TeacherId, StudentId)
);