is admin through memberships

is_admin.rb

class Member
  # ...
  class << self
    # ...
    def is_admin?(organization, user)
      member = first(:conditions => {:organization_id => organization.id, :user_id => user.id})
      return false unless member
      return member.role == 'admin'
    end
    # ...
  end
  # ...
end

class Organization
  # ...
  def user_is_admin?(user)
    Member.is_admin?(self, user)
  end
  # ...
end

class User
  # ...
  def is_admin_of_organization?(organization)
    Member.is_admin?(organization, self)
  end
  # ...
end