Avoid negative quantity

I'm storing a quantity of current items in [tbl_quantity].[quantity].  (key field [ID], related to [tbl_items].[id] via one-to-one with referential integrity)

When using Form FRM_ORDER, I don't want to allow the order to exceed the quantity.  (Maybe a message "There are only x quantity on hand, adjust your order now."

What's the easiest way to do this, or does someone have a very simple inventory system (office supplies, not sales) they would like to share?
TIgerVAsked:
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Hamed NasrRetired IT ProfessionalCommented:
Try this in Validation Rule for the required field in the form:

Example to check if value entered in f2 is not more than f1:

Validation Rule: <=[f1]
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TIgerVAuthor Commented:
No, that returns a very unintelligible error.
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Hamed NasrRetired IT ProfessionalCommented:
Can you reproduce the issue with a sample database?
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PatHartmanCommented:
Typically your quantity on hand will be stored in a separate table from the one that holds the transactions.  Many applications do not even store a quantity on hand but calculate it on the fly each time it is needed.  Sounds like you are using method 1.  I think this method is dangerous but you can use it if you are careful.  In the query for the form, join to the inventory table.  That gives you immediate access to the quantity on hand.

In the Form's BeforeUpdate event:
If Me.txtAmt > Me.qtyOnHand Then
    Msgbox  "You have requested more items than we have available.  Please correct.",vbokOnly
    Me.txtAmt.SetFocus
    Cancel = True
    Exit Sub
End If

Me.qtyOnHand = Me.qtyOnHand - Me.txtAmt
 

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The "If" will ensure that you don't request more than are available and exit the sub if you do.  The last statement will decrement the quantity on hand and Access will save the inventory record along with the transaction record.

Now - the reason this is dangerous is because you are almost certainly working in a multi-user environment.  There may be sufficient qty on hand when you look but someone else might save before you do and take that quantity away.  If someone else is working on the same item as you but managed to save before you did, you will get a very confusing error message giving you three options.  You need to instruct (and trust) your users to choose the discard option and start again with their transaction.
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TIgerVAuthor Commented:
Thank you!
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