Get data from another table in a ms Access data macro - sql

Question

I have a tblCustomer and a tblContact, where the information ultimately comes from two different other programs. In order to get around complex ID issues and keep things easy for users, I'm trying to do the following:
If either the CompMyobID or Company (lookup which points to the Primary Key autonumber in tblCustomer) field of a tblContact record are updated, the data macro finds the corresponding tblCustomer record, and uses the Primary Key or the MyobID to update the non-updated field in tblContact.
To achieve that, I've created the following after-update macro - First an If to work out if one of those fields (and which one) has been updated, then an EditRecord, alias not set in order to edit the current record where I SetField the field with the other name to... and that's where I get stuck. I tried to just run some sql, but at the moment nothing actually happens when I update either field...
Edit:
Because, as Erik pointed out, this method locks me out of the current record, and opens me up to recursive issues, Here's another angle that's not yet working:
in a Before Change Data Macro, the following:

Answer 1

You've got two main errors in that data macro:
The just updated record in an after update macro is read-only. This means calling EditRecord will fail. You can work around that by looking up the record you just updated, as shown here, but this requires you
You can't just set the value equal to an SQL clause, and expect the database to execute that clause. Usually, you could lookup records using DLookUp, but data macro's use the Look Up A Record In blocks to look up records instead
The final macro should look something like this (only updating one):
If Updated("CompMyobID") Or Updated("Company") Then
SetLocalVar
NewID
=[ID]
SetLocalVar
NewCompany
=Company
SetLocalVar
NewCompMyobID
=CompMyobID
If Updated("CompMyobID")
Look Up A Record In tblCustomer
Where Condition CompMyobID = NewCompMyobID
Alias T
SetLocalVar
LookupCompany
=T.ID
For Each Record In tblContact
Where Condition ID = NewID
EditRecord
SetField
Company
LookupCompany
End EditRecord
End If
End If
A strong warning: This macro will recursively call itself the way you have designed it! If you update Company, the macro will edit CompMyobID, triggering the macro, then updating Company again, triggering it again, etc.
Access has protection against recursive macro's, so aside from the errors you're triggering, you will probably not notice the recursion, and it won't cause performance problems. I strongly recommend not relying on this protection, and not storing information that could be looked up, but instead using a relation to look up the appropriate company name.
In the end, after the comment discussion below, We used a Before Change data macro, which avoids the need to look up the changed record before altering it, and avoids the chance of the macro calling itself because a Before Update macro edits the current record before it gets updated, instead of updating it again after it gets updated. We changed the macro to check if the value was Null or a zero-length string because Updated is not available in a before update macro.
The final solution was equal to the last edit of the question, with the exception of using Company & "" = "" instead of Company = "" to account for Null values.
You can read more on Null vs zero-length string here. The essence is that a zero-length string is just a string with 0 characters, but Null is a place where no value has been entered. Null behaves a bit oddly, returning Null when you compare it with anything. This means Null = "A" is Null, and gets treated as false, but Null <> "A" is also Null, and also gets treated as false.