Dr. Dobb's Journal - December 2008 - (Page 27)

d12weis_p4db 10/10/08 9:31 AM Page 27 Note that I’m calling #detect, not #find. This is because I want to invoke the method provided by the Enumerable module, and ActiveRecord::Base#find overrides Enumerable#find. This strategy works pretty well for a small number of commenters; however, #detect performs a sequential search, an O(n) operation. This method won’t perform well for a large value of n, when there are a lot of commenters. You might want to build a lookup hash for constant-time lookup, or O(1): # controller @commenter_lookup = post.commenters.inject(Hash.new) do |hash, user| hash[user.id] = user; hash end The view looks up the record for the comment’s user_id in the hash, and needs to call #first to pull the one and only record out of the array before getting the count. Code like this should be well commented, particularly the fact that it addresses a performance issue. There is another option for this kind of problem: denormalization. Solution 5: Denormalize “Normalize until it hurts. Denormalize until it works.” In a perfectly normalized database, there is only one representation of any particular fact. Taken to the extreme, this results in a space-efficient database with no chance of duplication or inconsistency. This is a wonderful ideal, but it comes at a cost: time-efficiency. We software developers walk a fine line between idealism and pragmatism. Active Record makes it very easy to denormalize the number of comments that Most unexpected performance challenges are related to querying the database Now the view can fetch the user for a given comment from the hash: # view @commenter_lookup[comment.user_id] Solution 4: Aggregate Another example of when the :include option isn’t enough is when you are fetching aggregated data. Let’s say you’d like to fetch the number of comments each user has created to display that next to the user’s name. If you calculate it user by user (via comment.user.comments.count), you’ll have another n+1 problem. One approach is to calculate the data for all relevant users in a single SQL query and build a lookup hash. # controller @comment_count_lookup = @post.comments.all( :select => 'user_id, COUNT(*) as num_comments', :group => 'user_id').group_by(&:user_id) This code, which could be in a controller or model, gets the number of comments for each user by grouping by user_id and counting the rows in each group with the SQL COUNT function. The #group_by method creates a hash where the key is the user_id (because it’s returned by the block), and the value is an array containing all items with that key. The view can use this hash to look up the count for a given user: # view December 2008 l www.ddj.com l Dr. Dobb’s Journal 27 http://www.accusoft.com/ddj http://www.accusoft.com/ddj 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

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