Open Popup in add or edit modes

[Buddy]

I have an Access 2010 DB where there is a 1:many relationship between two tables. 1:many is not the problem. 1 to 0 is the most common, to the extent that we do not want to see the nought side unless data is requested by the user clicking a button. A subform is not suitable. We want the user to see a popup if he requests it by clicking a button. If there is data the popup must display the record in edit mode, if there is no data the popup must accept a new record. That is causing the problem.
I have not worked with access for at least ten years, the data base has changed - and I am on a steep learning curve. I tried to do it with macros using a on error statement to catch a no data error, but it did not recognize the error. The record ID will always be on the main form.
Any suggestions will be appreciated, examples would be wonderful.
Buddy

[orange]

In a 1 to Many relationship, why would you expect only 1 record on the many side?

If there is data the popup must display the record in edit mode

Why couldn't there be 2 , 4, 9 records? Which one did you want to open for edit?

More specifics please.

[ItsMe]

In the popup form's load event you might try something like ...

Dim lngRecordCount As Long
Dim rs As DAO.Recordset
Dim strFormName As String

Set rs = Me.RecordsetClone
lngRecordCount = rs.RecordCount
strFormName = Me.Name

    If lngRecordCount < 1 Then
        Set rs = Nothing
        DoCmd.Close acForm, strFormName
        DoCmd.OpenForm strFormName, , , , acFormAdd, acDialog
    Else
        rs.Close
        Set rs = Nothing
    End If