how can I set foreign key in master table to NULL ? In original component TDBLookupComboBox there was a property NullValueKey. This was not ideal for me for various reasons. First reason: there is bug in standard implementation (at least in RAD Studio XE4) so after user cleared the selection he had to open and close combo box afterwards (without changing selection !). Second reason: I would prefer alternative solution more intuitive to user to enhance list of items with blank item located at the beginning of list. So I replaced the original component with TMS version and now I see there is not any NullValueKey property. I can't set the ItemIndex to -1, because the component internally changes this to ItemIndex 0. Its really sad because it seems that I had to use plain old combo box and handle manually all the events required.
We'll need to extend the component for this specific capability as this feature is currently unfortunately not built-in.