I have a table with four fields making up the primary key. I need to be able to change one of these values but I still need it to be part of the primary key. When I try to modify that value and a few others, it fails, saying that a different, non null element can't be null and, crucially, that the insert failed, where I was expecting an update

Dim Refund As New BookingDB.EntityClasses.TicketRefundEntity(RefundsList.Rows(i)("SheetRef"), _
    RefundsList.Rows(i)("BookingRef"), RefundsList.Rows(i)("TicketNumber"), RefundsList.Rows(i)("MachineNumber"))

Refund.Save() ' Debug - This save works

Refund.FullRefundReason = RefundsList.Rows(i)("FullRefundReason")
If cmbView.Text = "Un-Printed" Then
    Refund.JourneyCharge = RefundsList.Rows(i)("JourneyCharge")
    Refund.GrossRefund = RefundsList.Rows(i)("GrossRefund")
    Refund.Atocfee = RefundsList.Rows(i)("Atocfee")
    Refund.AgencyFee = RefundsList.Rows(i)("AgencyFee")
End If

If ChangePeriod Then
    Refund.Period = RefundsList.Rows(i)("Period")
    ' This is where I need to change the Sheet Reference. I need to keep
    ' this in the primary key so tickets can be resubmitted if rejected.
    If cmbSheet.Text = "New" Then
        Refund.SheetRef = RefundsList.Rows(0)("TicketNumber") & RefundsList.Rows(0)("MachineNumber")
    Else
        Refund.SheetRef = cmbSheet.Text
    End If
    SheetRef = Refund.SheetRef
End If
Refund.Save()

I am using Self Servicing with VS2005

When change the value of the field, does the PK Fields values correspond to a row in the database?

Make sure the entity.IsNew property is set to false, before the last save.

I added Refund.IsNew = False and now it fails with the error: "During a save action an entity's update action failed. The entity which failed is enclosed."

Any ideas on how to investigate this further? It is currently in a try loop. Would removing that help me to get further info?

It's better to examine the generated SQL Query if any. And then try to execute it direcly against the database. To get the generated query, please check the following manual's section: Using the generated code -> Troubleshooting and debugging"

Thanks Walaa. It turned out I had the primary keys in the wrong order when I was creating the instance of the entity. All sorted now.