Help on my validation

I use text field with auto complete for my authority_name field which
will be fetched from userinfo table and the field name is login.

Authority Name ------------
<%=text_field_with_auto_complete :userinfo, :login%>

this authority name will be stored in company table.
what my problem is i want to validate the above auto complete field…
i tried the validates presence of it gets me error.
i could not use as <%=f.text_field_with_auto_complete :userinfo,
what is the reason…pls guide me…on the validation