Dr. Dobb's Journal - December 2008 - (Page 28) d12weis_p4db 10/10/08 9:31 AM Page 28 Core Technology PERFORMANCE ON RAILS a user has created. Just enable a counter cache on the association: class Comment true end end end end def before_destroy # need to handle case where user deletes their first comment earliest = user.comments.first :conditions => ['id ?', self.id], :order => 'created_at' user.update_attribute(:first_comment_ at, earliest.created_at) end And create a migration to add an integer column named comments_count to the users table. Here’s a migration that adds the column and calculates the count for each user because our users have created comments before we added this denormalization: class AddUsersCommentsCount 0 User.reset_column_information User.all.each do |u| User.update_counters u.id, :comments_count => u.comments.count end end def self.down remove_column :users, :comments_count Every time users create a new comment, their comments counter cache is incremented, and if they delete a comment, the counter cache is decremented. Now the view can display the user’s comment count without any additional database queries: Conclusion Counter caches are a simple type of denormalization that is built into Active Record, but your requirements might be more complex. Let’s say you want to denormalize the date and time of the user’s first comment. I would use the before_create and before_destroy lifecycle hooks to keep the data in sync: # comment.rb def before_save # time can only move forward, so this is pretty simple user.update_attribute (:first_comment_at, Time.now) if user.first_comment_at.nil? Ruby the language and Rails the framework often take a beating from detractors on the question of performance. Ruby’s not in the running to be the fastest language, and it’s not even the fastest interpreted language. Rails isn’t the fastest framework. However, if Ruby or Rails is your bottleneck, consider yourself lucky! Most unexpected performance challenges are related to querying the database and aren’t detectable until the app has been in the wild, which is no different than any other software development framework. DDJ 28 Dr. Dobb’s Journal l www.ddj.com l December 2008 http://www.getunderstand.com http://www.getunderstand.com http://www.ddj.com
Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - December 2008 Dr. Dobb's Journal - December 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Conversations The Man Who Sold the Sky Performance on Rails LINQ-to-SQL and T-SQL A Remote Java RMI Registry Beyond B-Trees File Descriptors and Multithreaded Programs Effective Concurrency The Agile Edge Swaine's Flames Dr. Dobb's Journal - December 2008 Dr. Dobb's Journal - December 2008 - Dr. Dobb's Journal - December 2008 (Page Cover1) Dr. Dobb's Journal - December 2008 - Dr. Dobb's Journal - December 2008 (Page Cover2) Dr. Dobb's Journal - December 2008 - Dr. Dobb's Journal - December 2008 (Page 1) Dr. Dobb's Journal - December 2008 - Dr. Dobb's Journal - December 2008 (Page 2) Dr. Dobb's Journal - December 2008 - Dr. Dobb's Journal - December 2008 (Page 3) Dr. Dobb's Journal - December 2008 - Contents (Page 4) Dr. Dobb's Journal - December 2008 - Contents (Page 5) Dr. Dobb's Journal - December 2008 - Friday Night Fish Fry (Page 6) Dr. Dobb's Journal - December 2008 - Friday Night Fish Fry (Page 7) Dr. Dobb's Journal - December 2008 - Friday Night Fish Fry (Page 8) Dr. Dobb's Journal - December 2008 - Friday Night Fish Fry (Page 9) Dr. Dobb's Journal - December 2008 - Alia Vox (Page 10) Dr. Dobb's Journal - December 2008 - Alia Vox (Page 11) Dr. Dobb's Journal - December 2008 - Developer Diaries (Page 12) Dr. Dobb's Journal - December 2008 - Developer Diaries (Page 13) Dr. Dobb's Journal - December 2008 - Conversations (Page 14) Dr. Dobb's Journal - December 2008 - Conversations (Page 15) Dr. Dobb's Journal - December 2008 - The Man Who Sold the Sky (Page 16) Dr. Dobb's Journal - December 2008 - The Man Who Sold the Sky (Page 17) Dr. Dobb's Journal - December 2008 - The Man Who Sold the Sky (Page 18) Dr. Dobb's Journal - December 2008 - The Man Who Sold the Sky (Page 19) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 20) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 21) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 22) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 23) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 24) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 25) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 26) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 27) Dr. Dobb's Journal - December 2008 - Performance on Rails (Page 28) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 29) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 30) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 31) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 32) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 33) Dr. Dobb's Journal - December 2008 - LINQ-to-SQL and T-SQL (Page 34) Dr. Dobb's Journal - December 2008 - A Remote Java RMI Registry (Page 35) Dr. Dobb's Journal - December 2008 - A Remote Java RMI Registry (Page 36) Dr. Dobb's Journal - December 2008 - A Remote Java RMI Registry (Page 37) Dr. Dobb's Journal - December 2008 - A Remote Java RMI Registry (Page 38) Dr. Dobb's Journal - December 2008 - A Remote Java RMI Registry (Page 39) Dr. Dobb's Journal - December 2008 - Beyond B-Trees (Page 40) Dr. Dobb's Journal - December 2008 - Beyond B-Trees (Page 41) Dr. Dobb's Journal - December 2008 - File Descriptors and Multithreaded Programs (Page 42) Dr. Dobb's Journal - December 2008 - File Descriptors and Multithreaded Programs (Page 43) Dr. Dobb's Journal - December 2008 - File Descriptors and Multithreaded Programs (Page 44) Dr. Dobb's Journal - December 2008 - File Descriptors and Multithreaded Programs (Page 45) Dr. Dobb's Journal - December 2008 - Effective Concurrency (Page 46) Dr. Dobb's Journal - December 2008 - Effective Concurrency (Page 47) Dr. Dobb's Journal - December 2008 - Effective Concurrency (Page 48) Dr. Dobb's Journal - December 2008 - The Agile Edge (Page 49) Dr. Dobb's Journal - December 2008 - The Agile Edge (Page 50) Dr. Dobb's Journal - December 2008 - The Agile Edge (Page 51) Dr. Dobb's Journal - December 2008 - Swaine's Flames (Page 52) Dr. Dobb's Journal - December 2008 - Swaine's Flames (Page Cover3) Dr. Dobb's Journal - December 2008 - Swaine's Flames (Page Cover4)
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