if with search fields you mean fields in the search bar this type of constraint is then not possible. If you're talking about fields on a form/page (dataview) you could give the lazy reference selector from the app store a try.
Thanks a lot guys, it is indeed at the search fields level of a template grid I need this. You cannot mean this, tha it is not possible in a "natural way" like the constrained by option in the case of a dataview drowpdown field. What a disappointment that is;-(! Using a separate entity sounds as a contrived workaround. I think the treeview widget is then a better option.