When making a query how do you check if the default value was returned?

Question

Using the code below:

let student =
    query {
        for student in db.Student do
        where (student.StudentID = 1)
        select student
        exactlyOneOrDefault
    }

How do you check the value of student, and just what would be the default value if no match in the database was found?

Answer 1

Database objects come from the database library you're using (either LINQ-to-SQL or Entity Framework), they're not originated from F# so they're nullable. The value returned by exactlyOneOrDefault when there isn't exactly one is null.