Record retrieval problem

[Novice programmer]

I have the following code snippet:

sql = "select * from customer where code = 'abc'"
conn.BeginTrans
With rs
.Open sql, conn, adOpenDynamic, adLockPessimistic,
adCmdText
If Not .EOF Then
' --- Force edit mode ---
!Code = "def"
Else
' --- Add key field(s) first ---
.AddNew
!Code = "def"
End If
End With

and run the program twice to test the locking function.
Suppose the 1st instance is editing the record 'abc'.
When the 2nd instance tries to retrieve the same record at
the Open statement, no error occurs and instead set the
EOF property to True automatically, making the program
thinks that there is no such record. Can I make the Open
statement fails with error when there is another user
editing
the same record? Any help or suggestion will be
appreciated.
Thanks!