Before Update

[samotek]

Io my main form i have a control called ChangePrice and on clkcing i
set the focus
to the control UnitPrice on the subform and enable the editing of the
price.
It cannot work because of the code in the BeforeUpdate event of the
subform which is as follows:

Private Sub Form_BeforeUpdate(Cancel As Integer)
If IsNull(Me.Quantity) Then
MsgBox " You have to fill in either quantity or cartons!", vbCritical
Cancel = True
DoCmd.GoToControl "productid"
Exit Sub
End If
End Sub
Thus,when i click the control Changeprice on the main form i receive
the message
" You have to fill in either quantity or cartons!", vbCritical,from the
BeforeUpdate event.
Can i restrict the action of the Before update event when i click the
control ChangePrice ?
for example
If Parent!CmdChangePrice = true Then
If IsNull(Me.Quantity) Then
MsgBox " You have to fill in either quantity or cartons!", vbCritical
Cancel = True
DoCmd.GoToControl "productid"
Exit Sub
End If
End If

I have tried it but without succes.Where i am wrong ?

==================================
I am trying to resolve it by putting a condition in the Before Update
event:
If Not IsNull(Me.Parent.CmdChangePrice) And IsNull(Me.Quantity) Then
......

But it doesn help.May be i have written it wrong