Forms :: Cascading Form - Numerical Order ID

I have a problem with a cascading form, which writes back to another table. The scenario is:

There are three tables. Users, Departments & SubDepartments.

Each table has an Autonumber set as the primary key .

The form is to write back to the user table, and within the user table there is a Department & SubDepartment field. These are linked to the relevant tables and all that works.

For the form I have two combo boxes for Department and SubDepartment. Department simply pulls from the Departments table and displays/writes back to User table correctly.

The SubDepartment is the one I am having problems with. I have got it to cascade correctly but here is the exact issue:

On the existing records the subdepartment is being displayed as a number (the primary key autonumber). The drop down list displays the text of what the subdepartment is (which is what I require) but then when that is selected it throws up the error that the value entered isn't correct for that field. I guess it's because it wants to write back the numerical record ID rather than the text from the other field.

The row source query is SELECT [qrySubDepartment].[SubDepartmentName] FROM qrySubDepartment ORDER BY [SubDepartmentName];

The table fields for the subdepartment table are SubDepartmentID, SubDepartmentName, DepartmentID and SDID.

SDID can be ignored for the purpose of this question and DepartmentID links back to the main department. Just to confirm visually the cascading form does work in terms of when you pick a department it only displays the subdepartments associated to it.