Thanks for this lesson, it is really helpful when it works.  I'm experience problems with it continuing to work, however.  

I've set up this code exactly as described and it works great.  Then I save, close and reopen the next day and attempting to run again I get bug errors.  I redo the entire process in a brand new set of forms, and again works great.  Next day, bug (error 459, pointing to the default input of the "personID" from the underlying form for the reverse relationship), which then leads to a bug in looking up the opposite ID as well.  

I'm not sure what the problem is, but I'm not making any edits to the forms or code in question, it simply seems to just stop working over time and I'm not sure why.

Best,
Jack

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Reply from Alex Hedley:

If you compile the code in the VBA editor do you get any errors?
Might be worth a compact and repair.
Have you tried deleting and recreating the form?