Accessing the relationship of a relationship with Entity Framework

Question

I the School class I have this code:

from student in this.Students where student.Teacher.Id == id select student

The Student class there are two relationships: Teacher and School. In the School class I'm trying to find out all the students whose Teacher has a given id.

The problem is that I get

System.NullReferenceException: Object reference not set to an instance of an object.

in the statement

student.Teacher.Id

I thought of doing this.Students.Include("Teacher"), but this.Students doesn't have such a method. Any ideas how can I perform that query?

Answer 1

The Include clause can be used in linq to entities query. For example:

using (YourDataContext db = new YourDataContext())
        {
            var query = from s in db.Students.Include("Teacher")
                        where s.Teacher.ID == 1
                        select s;
             //etc...
        }

I guess that this.Students is collection pulled into memory already, so you might consider this code in the part you are retrieving students from dbs. If you want to load Teacher of the Student later on ( very importand is that student entities are tracked by ObjectContext!) you can use the Load mehtod on TeacherReference property, but for every student in this.Students collection separately:

student.TeacherReference.Load();

Hope this helps

Answer 2

You show this line of code:

from student in this.Students where student.Teacher.Id = id select student

First, the = should be a ==

Is that just a typo?

Second, you don't need Include for the following, corrected query, if you don't dereference Teacher later on:

var q = from student in SomeObjectContext.Students 
        where student.Teacher.Id == id 
        select student;

LINQ to Entities doesn't require Inlcude for just the where clause.

You would need Include if you later iterated the results and dereferenced Teacher:

foreach (var student in q)
{
    Console.WriteLn(student.Teacher.Id);
}

Third, you show this error:

System.NullReferenceException: Object reference not set to an instance of an object.

This is not a LINQ to Entities error. Is the error actually on that line of code, or is it somewhere else?

Also, what's this? If it's not an ObjectContext, then you are in likely LINQ to Objects, not LINQ to Entities. In that case, you wouldn't have Include available. If this is the case, then where does this.Sbudents come from?

Answer 3

I've found the debugger let me walk throu each iteration of the query:

from student in this.Students where student.Teacher.Id == id select student

so I've got to see student.Teacher.Id == id many times. Every time I was debugging it didn't happen and everything worked just fine. I turned the expression into:

from student in this.Students where student.Teacher != null && student.Teacher.Id == id select student

and it not only stopped crashing, but it worked fine as well (all students where selected as expected). I'm not totally sure why.