Question

I have this small application that I’m building as an exercise to learn the basics of the Entity Framework. It uses a MySQL db with 3 tables: Courses, Students and Students_has_Courses:

I used this db to create an Entity model in Visual Studio:

It’s working fine. I can bind a table with my datagridview, modify data and press a button to save the changes. But as you see, the Students_has_Courses is an association (this is pretty new to me). And now my question: I need every Course ID for a specified student ID (to know which courses a student is taking). I thought this LINQ query would be fine:

var query = from s in school.Students.AsEnumerable()
            where s.ID == selectedStudentId
            select s.Courses;

But I can’t really seem to extract the Course ID’s from this EntityCollection ? I used a foreach(var course in query) but I’m really stuck here.

Answer 1

You should be able to get the course IDs by using:

var studentsCourseIDs = (from s in school.Students
                         where s.ID == selectedStudentId
                         select s.Courses.Select(c => c.ID))
                        .Single();

Alternatively you start from the Courses set:

var studentsCourseIDs = (from c in school.Courses
                         where c.Students.Any(s => s.ID == selectedStudentId)
                         select c.ID)
                        .ToList();

Don’t use the AsEnumerable() in your example because it will load the whole students table into memory before the where clause and the selection is applied.