Entity Framework 4, inserting with foreign key value not ID

问题

This is pretty basic question I am sure but it's baffling me now :S

I have 2 tables, Students and Courses with a foreign key constraint, Many Students to 1 Course.

I store in my Students table a CourseId_FK reference, so when I am creating a Student how would I be able to do something like this?

Students student = new Students();
student.Name = Bob;
student.Course.CourseName = "Geography";
db.SaveChanges();

Right now, the above doesn't work and I have to resolve to

student.CourseId = 22;

Can anyone help me pls?

Thanks

David

回答1:

If you don't want to load the full course entity:

Student student = new Student();
student.Name = Bob;
student.CourseId = db.Courses
                     .Where(c => c.CourseName == "Geography")
                     .Select(c => c.CourseId).First();
db.SaveChanges();

This would only fetch the Id of the Geography course from DB (or crash when there isn't any).

回答2:

Looking at your entities, does "Student" have a "Course" (Really: Courses) property?

Your entity should probably look something like this, for what you want to do:

public class Student
{
    public string Name {get; set;}

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

Then you'd do something like:

Student student = new Student();
student.Name = Bob;
student.Courses.Add(db.Courses.Where(c => c.CourseName == "Geography"));

Edit: If the Student can only have one course, you modify the Student class so it looks like this:

public class Student
{
    public int StudentId {get; set;}
    public string Name {get; set;}
    public int CourseId {get; set;}
    public virtual Course Course {get; set;}
}

Then your code looks like this:

student.Course = db.Courses.Where(c => c.CourseName == "Geography"));

What's going on with your code (I expect) is that you only have the CourseId so when you're so you can't assign an actual course object to your student.

回答3:

I'd do something like this:

Student student = new Student();
student.Name = Bob;
student.Course = db.Courses.First(c => c.CourseName == "Geography");
db.SaveChanges();

Update: the answer is about the situation when a student has zero or one course (as in the provided code).

来源：https://stackoverflow.com/questions/6113570/entity-framework-4-inserting-with-foreign-key-value-not-id

标签

entity-framework

foreign-key-relationship