gistfile1.rb

# MODEL

class Theater
  named_scope :with_recent_comments, 
    :select => "*, recent_comments.created_at AS last_comment_at",
    :joins  => "INNER JOIN (SELECT id, entity_id, created_at FROM comments WHERE entity_type = 'Theater' LIMIT 100) 
                AS recent_comments ON recent_comments.entity_id = theaters.id",
    :order  => "recent_comments.created_at DESC"
end

# CONTROLLER

Theater.with_recent_comments

# ------------ OR ---------------------

# MODEL

class Comment
  named_scope :for_theaters, :conditions => {:entity_type => "Theater"}
  named_scope :recent, :limit => 100
end

class Theater
  def self.with_recent_comments
    recent_comments = Comment.recent.for_theaters.all(:select => "DISTINCT entity_id", :order => "id DESC")
    find(recent_comments.map(&:entity_id)).sort_by {|t| recent_comments.index(t.id)}
  end
end

# CONTROLLER

@theaters_with_comments = Theater.with_recent_comments

# VIEW

%h2 Recent comments
%ul
  - @theaters_with_comments[0..9].each do |theater|
    %li= link_to theater.name, theater_path(theater)