WCF Data Services - neither .Expand or .LoadProperty seems to do what I need

Question

I am building a school management app where they track student tardiness and absences. I've got three entities to help me in this. A Students entity (first name, last name, ID, etc.); a SystemAbsenceTypes entity with SystemAbsenceTypeID values for Late, Absent-with-Reason, Absent-without-Reason; and a cross-reference table called StudentAbsences (matching the student IDs with the absence-type ID, plus a date, and a Notes field).

What I want to do is query my entities for a given student, and then add up the number of each kind of Absence, for a given date range. I prepare my currentStudent object without a problem, then I do this...

Me.Data.LoadProperty(currentStudent, "StudentAbsences") 'Loads the cross-ref data

lblDaysLate.Text = (From ab In currentStudent.StudentAbsences Where ab.SystemAbsenceTypes.SystemAbsenceTypeID = Common.enuStudentAbsenceTypes.Late).Count.ToString

...and this second line fails, complaining "Object reference not set to an instance of an object."

I presume the problem is that while it DOES see that there are (let's say) four absences for the currentStudent (ie, currentStudent.StudentAbsences.Count = 4) -- it can't yet "peer into" each one of the absences to look at its type. In fact, each of the four StudentAbsence objects has a property called SystemAbsenceType, which then finally has the SystemAbsenceTypeID.

How do I use .Expand or .LoadProperty to make this happen? Do I need to blindly loop through all these collections, firing off .LoadProperty on everything before I can do my query?

Is there some other technique?

Answer 1

When you load the student, try expanding the related properties.

var currentStudent = context.Students.Expand("StudentAbsences".Expand("StudentAbsences/SystemAbsenceTypes").Where(....).First();