Count records in subform

I have a form that contains a textbox called "RejectNum". This form is bound to a table called Reject Main.  I have a subform bound to this form called RejectSub, that is used to enter multiple rejects and their various criteria.  The datasource for the subform is a table called RejectSub, and there is a one to many relationship between the two tables based on a key called UID. (probably useless info, but maybe worth adding).

Each reject has a serial number, and it is entered in a textbox called "SerialNum".  I would like to force a piece of logic to prompt a msgbox when the number of rejects entered on the main form ("RejectNum") does not equal the count of serial numbers entered on the subform.  In other words, if the number in the RejectNum box is 20, users should have to enter 20 serial numbers in the subform, and until they do so, they should not be able to add a record.  Can someone help me with the logic here?

Thanks in advance for your help.  Please let me know if the explanation has been confusing or otherwise inadequate.

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ala_frostyConnect With a Mentor Commented:
I don't think this is the best way to go about getting what you want.

Here's what I would do:

* Delete the field in the RejectMain that has the count of the records in RejectSub. It's redundant.

* in the textbox RejectNum, change the source to
= [sfmRejectSub].form.recordcount

* If you want, before saving the record, prompt the user to verify that s/he there were [sfmRejectSub].form.recordcount Rejects. Users tend to quickly tire of this sort of relentless asking for verification.

* go into the Database Relationships, and set up a cascaded enforced relationship between tblRejectMain and tblRejectSub

The advantage to doing it this way is that in the event of something crashing Access (and who ever heard of that) you have the data you entered in both the Main and the Sub.

There is only so much you can do to prevent the users from making mistakes. Yes, don't make it easy, but the way you were suggesting going about it seems painful to me.
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