[TFUI] Test Driven Wizardry

TFUI:

My research has made me an expert at writing technical questions on
the MS newsgroup that professionals and MVPs cannot answer.

Today's installment is called "Test Driven Access Development". I'l
copy it here so we can at least discuss the ramifications, if not the
actual technical answer:

The canonic way to write "unit" tests for a database is this:

  setUp
      begin a transaction
      create Sample Database

  test_foo
        select stuff in the database
        assert it matches the samples

  test_bar
        update stuff in the database
        select it back
        assert it matches

   tearDown
        rollback the transaction

That's relatively efficient because transactions buffer everything
with miminal overhead. I want to do that to Access while I test a
Form. And I want to do it from VB Classic, not from VBA inside Access.

So let's set up an experiement to see how close we can get to that
pattern. Creae a form, paint a button, set up the References to Access
and DAO, and write this:

Private Sub Command1_Click()

    Dim ax As New Access.Application
    ax.Visible = False
    ax.AutomationSecurity = msoAutomationSecurityLow
    ax.OpenCurrentDatabase "database.mdb"
    Dim cn As DAO.Database
    Set cn = ax.CurrentDb
    ax.DBEngine.BeginTrans

 ' beginning to build the sample database
    cn.Execute "DELETE FROM DataTable"

    ax.DoCmd.OpenForm FormName:="DataTable"

End Sub

The last line fails with the infamous 3008 error message, "The table
'DataTable' is already opened exclusively by another user, or it is
already open through the user interface and cannot be manipulated
programmatically."

The .BeginTrans call did it. Without that line, I can delete the table
and open its form fine. (I intend to write test cases that mess with
that form, and its data, for the usual test-first benefits.)

The standard answer to 3008 is to turn off record locking. All our
locking here is off and optimistic.

To workaround, I can of course just fudge. I could manually build a
transaction by copying the DataTable to a backup table, then restore
it during tearDown(). Or I can let the test data polute the golden
copy of the database.

The question is this: Am I missing some tweak in the interaction of
BeginTrans and the Form architecture where I can have a real
transaction, and the Form can display data from that transaction
transparently?

-- 
  Phlip
  http://www.c2.com/cgi/wiki?ZeekLand


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