collection.rb

class PersistentCollection
  def initialize(klass)
    @klass = klass
  end

  # ...
end

# This should be reusable
class ScopedCollection
  def initialize(collection, scope = IdentityScope.new)
    @collection = collection
    @scope      = scope

    super @collection
  end

  def each(&block)
    @scope.on_read(@collection).each &block
  
    self
  end

  def <<(e)
    @scope.on_write e

    @collection << e

    self
  end
end

# This is the business-specific part
class PublishedPostScope
  def initialize(publisher)
    @publisher = publisher
  end

  def on_read(collection)
    collection.where('published_at is not null and publisher_id = ?', publisher.id)
  end

  def on_write(post)
    post.published_at = Time.now
    post.publisher = @publisher
  end
end

# And this is how you use it
pp = ScopedCollection.new(PersistentCollection.new(Post), PublishedPostScope.new(current_user))