gistfile1.rb

# Is this crazy?

class AccountRole < ActiveRecord::Base
  belongs_to :account
  belongs_to :target, :polymorphic => true

  validates_presence_of :role
end

class Project < ActiveRecord::Base
  has_many :account_roles, :as => :target
  has_many :admins, :through => :account_roles, :source => :account, :conditions => {:account_roles => {:role => "admin"}} do
    
    # This might be the crazy bit.  Or it might be the genius bit.  I'm not sure.
    def add(account)
      AccountRole.create!(:role => 'admin', :target => proxy_owner, :account => account)
    end

    def remove(account)
      # Note, scoping should automatically fill in the target conditions
      AccountRole.destroy_all(:conditions => {:account_id => account_id, :role => 'admin'})
    end
  end
end

# Usage:

Project.first.admins.add(an_account)