> Test whether using the full recommended syntax for KeyDescLinks changes the
> behaviour: set it as PCQUERY.MPNAME=MPNAME. Also, make sure that the
> table MAINPRODUCTGROUPS has no records with null in MPNAME, since the
> KeyDescLinks matching won't match null to null.

I have used the recommended syntax and MPNAME is not NULL, but the same
behaviour. I have done some debugging in the IBO-Code and I have a
guess, what the reason could be.
The procedure TIB_Dataset.SysUpdateKeyLinksData in the IB_Components.pas
has the following code:

...
if KeyChildUpdatingLevel = 0 then
begin
wasExcp := false;
BeginKeyDataFreeze;
KeyDataset.Fields.BeginUpdate;
try
try
for ii := 0 to KeyLinks.Count - 1 do
begin
tmpCanModifyKeyLinkField := true;
tmpCol := FieldByName( KeyLinks.IndexNames[ ii ] );
for jj := 0 to MasterLinks.Count - 1 do
if tmpCol = FieldByName( MasterLinks.IndexNames[jj] ) then
begin
tmpCanModifyKeyLinkField := false;
Break;
end;
if tmpCanModifyKeyLinkField then
KeyDataset.FieldByName( KeyLinks.IndexValues[ ii ]
).Assign(
FieldByName( KeyLinks.IndexNames[ ii ] ));
end;
if KeyDataset.NeedToPost then
for ii := 0 to KeyDescLinks.Count - 1 do
KeyDataset.FieldByName( KeyDescLinks.IndexValues[ ii ]
).Assign(
FieldByName( KeyDescLinks.IndexNames[ ii ] ));
except
wasExcp := true;
raise;
end;
finally
KeyDataset.Fields.EndUpdate( true ); <-------------
EndKeyDataFreeze;
if wasExcp then
SysKeyDataChange( nil );
end;
end;
...

I have changed the marked line from KeyDataset.Fields.EndUpdate( true );
to KeyDataset.Fields.EndUpdate( false ); and now it works. The
KeyDescLinks are the last values, that are assigned. And because the
SingleEventOnly-parameter of Fields.EndUpdate is set to true, the
changes to the KeyFields are suppressed. But why in the
OnDataChange-Event the field-parameter null is, is not clear to me.

Regards

Guido