Set a Trailer Check Id Macro

[Ann]

I need help with setting up a macro to check Trailer In
Id# on a form/with a subform embedded.
The Trailers get Checked In on a Check In form and then
get Checked Out on a Update Time Out form with a unbound
combox field.

The users are entering duplicate Tralier numbers for the
same trailer into the database on the Trailer Check In
form.
I'd like to set up a macro that upon Trailer Check In,
there would be a way to go to the embedded subform,
Tralier Date and Time Out to check the field Date and Time
Out on the last time the Tralier was Checked In, if it "
Is Null", then NOT allow them to enter the Trailer number
into the Check In form. Instead prompt them with a message
that the Trailer needs to be Checked Out before entering
into the database again.
Thanks in advance.

 

[Ann]

Need help with similar situation .......
I have a FORM, which contains a SUBFORM. I want to be
able to check the Trailer Id number in the FORM against
the data in the SUBFORM. And if they don't match ...
BEEP , MSGBOX , and force the user to enter Check Out
Trailer Date and Time in the Update Trailer Check Out form
before creating another new Trailer Check In entry.
Thanks,