Seems Complex validation --- pls help

Hi..
i have a auto complete field for my authority_id field.authority_id
field is not mandatory.if the user not fills it will take the default
authority id as value.
but i want to validate if the user enters invalid authority name into
the auto complete text box...
i can do this in my controller...
but how can i implementd as modal validation.
pls help on this
below is my controller side implementation
#if autority text box is empty?
if params[:user_info][:login].empty?
              @user = UserInfo.find_by_login('admin')
              @company.authority_id = @user.id
else
     authority = UserInfo.find_by_login(params[:user_info][:login])
     if authority.nil?
        flash[ :error ] = "*Please Enter valid Authority name"
     else
         @company.authority_id = authority.id
     end
end

hi..
some one pls help me...
thanks