Rails - Aug 2006 - How to create/save unary (pk-only) models

I have a table that''s currently only a primary key.  Basically:

   create_table "entites", :force => true do |t|
   end

class Entity < ActiveRecord::Base
   has_many :entity_members
end

   create_table "entity_membership", :force => true do |t|
     t.column "person_id", :integer, :null => false
     t.column "role", :string, :null => false
     t.column "entity_id", :integer, :null => false
   end

class EntityMembershp < ActiveRecord::Base
   belongs_to :entity_member
   belongs_to :person
end

The entity id is needed to link the aggregation to various other  
models, but there''s no other information that can or should be  
associated with it.

However I can''t seem to get the Entity class to create/save as it  
doesn''t seem to auto-assign the id.  From the console I''ve
tried:
 >> a = Entity.new
=> #<Entity:0x250fac0 @new_record=true, @attributes={}>
 >> a.save
ActiveRecord::StatementInvalid: PGError: ERROR:  syntax error at or  
near ")" at character 22
: INSERT INTO entities () VALUES()
[long error backtrace snipped]
 >> a = Entity.create
ActiveRecord::StatementInvalid: PGError: ERROR:  syntax error at or  
near ")" at character 22
: INSERT INTO entities () VALUES()

Is there anyway I can make this work?  Is there anything cleaner than  
adding a dummy column?

Eric