Adding "groups" to a user model

Rails
1

At present I have the following models:

users <-> permissions <-> objects

I wish to expand this to include groups, so that groups can contain
users and have permissions (a user might be part of more than one
group). However, the only way I see to do this is:

users <-> user_permissions <-> objects
users <-> group_memberships <-> groups <-> group_permissions <-> objects

Is there a nicer way to do this so that I only need one permissions
model, or is this the best way to do it?

Also, whichever way I do it, is there a nice way to make groups for
special cases such as “all users” and “administrators” without having
them in the database?

Thanks.

2

if you look at the code from the Rails Recipes book (
http://www.pragmaticprogrammer.com/titles/fr_rr/code.html) you’ll find a
potential solution in Recipe 32. It’s a bit lengthy to go into, and I’m
not
that hot at summarizing code, but basically you have this:

class User < ActiveRecord::Base
has_and_belongs_to_many :roles

other stuff

end

class Role < ActiveRecord::Base
has_and_belongs_to_many :users
has_and_belongs_to_many :rights
end

class Right < ActiveRecord::Base
has_and_belongs_to_many :roles
end

the rights table contains a field for name, controller, and action