gistfile1.rb

context "with associations" do
  setup do
    fake_association = Struct.new(:macro, :name, :object)
    fake_object_with_associations = Struct.new(:associations) do
      def reflect_on_all_associations
        associations
      end
      
      def method_missing(meth, *args, &blk)
        association = associations.find{|a| a.name == meth}
        association && association.object
      end
      
      def respond_to?(meth)
        associations.find{|a| a.name == meth}
      end
    end

    associations = []
    associations << fake_association.new(:belongs_to, :user, @instance)
    
    @post = fake_object_with_associations.new(associations)
  end
  
  should "have has_role? method using association if association exists" do
    assert @instance.has_role?(:user, @post)
  end
end

gistfile2.rb

def has_role?(role, object = nil)
  is_active_record_object = object.class.respond_to?(:reflect_on_all_associations)

  role = role.to_s
  plural_role = role.pluralize

  if is_active_record_object
    associations = Hash[*object.class.reflect_on_all_associations.map{|a| [a.name, a.macro]}.flatten]
    match_association = associations.keys.find{|a| a == role.to_sym || a == plural_role.to_sym}
    match_association_type = associations[match_association]
  end

  if is_active_record_object && match_association
    case match_association_type
    when :has_many, :has_and_belongs_to_many
      begin
        !!object.respond_to?(plural_role) && object.send(plural_role).find(self.id)
      rescue ActiveRecord::RecordNotFound
        false
      end
    else
      object.respond_to?(role) && object.send(role) && object.send(role).id == self.id
    end
  else
    self.send("#{role}?")
  end
end