Rails - May 2006 - Single table lookups

I''m struggling with a single table type (self) lookup.

my ''personnel'' model has a supervisor_id and is_supervisor
column.

The supervisor''s select list is created by
  @supv = Personnel.find(:all,
   :conditions => ["is_supervisor = true"],
   :order => ''last_name'')

in my personnel model, I have the following...

  def supervisor
    if Personnel.find_by_id(self.supervisor_id) == nil then
      Personnel.find_by_id(self.id).suwholename
    else
      Personnel.find_by_id(self.supervisor_id).suwholename
    end
  end

where ''suwholename'' is an aggregate value of the first,
middle, last
names.

The following view code produces a select list of supervisors...

  <%= options = [[''Select'', '''']] +
@supv.collect {
     |supv| [supv.suwholename, supv.id] }
     select ''personnel'', ''supervisor_id'',
options %></TD>

but I have 2 problems...

1. If there is no supervisor_id, I get an error...
   PGError: ERROR:  invalid input syntax for integer: ""
   : SELECT * FROM personnels WHERE (personnels."id" =
'''' )  LIMIT 1

2. When I have a supervisor_id, it doesn''t come up as
''selected'' in the select list.

I have struggled for a while with this and can''t seem to make any
headway.

Craig