Linq groupby with joining table

Question

Let's suppose there is a database to manage students. I'm using the following (simplified) classes:

Student
int id
string name

StudentEnrollmentInfo
int studentId
int courseId

Course
int courseInfo
string name

In the aforementioned tables, studentenrollmentinfo is a joining table, to handle the many to many relation between courses and students.
Now I'd like to query the names of all students who are currently enrolled in a list of different courses. For example:

Get all students enrolled in Biology, Maths and Arts, I'd like to get all students that are enrolled in ALL of those classes.

I have tried the following:

List<string> requiredCourses = new List<string>(){"Maths", "Biology", "Arts"}; 

var query = from student in _context.students
join enrollmentInfo in _context.StudentEnrollmentInfo on 
    student.id equals enrollmentInfo.studentId
join course in _context.Course on course.id equals enrollmentInfo.courseId
where requiredCourses.All(r => r.equals(course.name))
select(student.name)

This however does not work, my guess is because the joins, in addition to the joining table flattens the list, making it where every student with every course combination is a unique value, making it so that there is never any AND condition that is true.

Can anyone suggest something to point me in the right direction?

Answer 1

List requiredCourses = new List() { "Maths", "Biology", "Arts" }; var query = (from student in _context.students join enrollmentInfo in _context.StudentEnrollmentInfo on student.id equals enrollmentInfo.studentId join course in _context.Course on course.id equals enrollmentInfo.courseId where requiredCourses.Contains(course.name) select student.name).Distinct().ToList()

hope it will solve your purpose. distinct will help for your unique data