Entity Framework partial classes with custom attributes

How should I handle a custom property in a situation where I use partial classes with the EF-generated classes?

Here's the setup:

TimeSheet Table - this stores an employee's hours
- TimeSheetID (auto, int, PK)
- EntryDate (DateTime)
- Hours (int)
- EmployeeID (int)

EmployeeHourlyRate table - this stores an employee's current hourly rate. Historical rates are stored here as well.
- RateID (int, PK)
- EffectiveDate (int, PK)
- Rate (double)

This is a one to many relationship from TimeSheet to EmployeeHourlyRate. In order to find an Employee's rate, I would select the max effectiveDate less than the timeSheet's EntryDate.

In order to facilitate things, I've made a partial class called TimeSheet and added a new property called "Rate" to that class. What I would like to do is populate that myself from the same query that populates my collection of TimeSheets. I just know of no easy and clean way to handle this.

For example, I could do it this way:

var list = from ts in Context.TimeSheets
          ....
          select new TimeSheet() {
                 TimeSheetID = ts.TimeSheetID,
                 EntryDate = ts.EntryDate,
                 Hours = ts.Hours,
                 EmployeeID = ts.EmployeeID,
                 Rate = SomeRate   //real code has been omitted 
          };

This in theory should work, but for some reason the EF complains at run-time that I'm re-using an entity generated class (no idea why -- it works fine if I create my own custom class). However, even if it did work, I've still got to maintain a list of fields and keep on mapping from my EF to a single class -- i.e, maintenance becomes a problem when/if I add new fields to the TimeSheet table. It is also silly to have to re-type all that info.

So my question is, how do people generally handle this scenario? Is there a way to do something in the datamodel that would be able to effectively know my join rule (about selecting the correct effective date based on my EntryDate) and handle this?