Rails 3 scope options to consider...

rails_3_scope_options.rb

scope :latest_by_hashtag, lambda {|hashtag| {
  :where => { :hashtags => hashtag },
  :order_by => :tweeted_at.desc,
  :limit => 20 }
}

# OR #

def self.latest_by_hashtag hashtag
  where(:hashtags => hashtag).order_by(:tweeted_at.desc).limit(20)
end

# OR #

module Scopes
  def latest_by_hashtag hashtag
    where(:hashtags => hashtag).order_by(:tweeted_at.desc).limit(20)
  end
end
extend Scopes