Active Record No Nos
Use your common sense

About me
Patrick Hüsler
Freelance developer
Apple enthusiast
Surfing and Kung Fu

AR Kool Aid
No giant xml mapping file
Dynamic finders
Associations
Inheritance
Polymorphism
...

IT’S ALMOST WIZARDRY!!!!

BUT...

SO...

Be reasonable or ...

Code like this
@user.friends.map {|friend| friend.id}

Or this
@user.friends.sort_by{ |friend|
[ friend.online? ? 1 : 0,friend] }

Or this
friends.accepted.map{ |friend|
friend.logs.visible_for(self) }.flatten

eventually leads to

THIS

We can do better ...
Look at the generated SQL
Create indices
Let AR/SQL do the math (conditions,count,sum,
named_scope, ...)
Paginate, but paginate in SQL not on ruby collections
Only fetch what you really need (:select => :id)
Use :include (n + 1 problem)

Check SQL logs
tail -f log/*.log
Log to STDOUT in script/console
Do it once in ~/.irbrc
Use one of the query analyzer plugins
Use profiling tools like New Relic RPM, FiveRuns
TuneUp or Rack::Bug (Check railscast)

Where to look
Iterations/calculations in views / partials
Calculations on potentially big collections in ruby that
can be done in SQL
@user.friends.recent.sort_by{ |friend| [ friend.online? ?
1 : 0,friend] }
Pagination on collection instead of SQL
User.all.paginate(:per_page => 1, :page => 1)

Where to look
Not eager loaded associations
Post.all.each{|post| puts post.author.name}
Classic n + 1 problem
Post.all(:include => :author)
Ruby is used for tasks that could be done with SQL
Post.all.map(&:id)
Instead of Post.all(:select => :id)

Image ressources
http://www.gunaxin.com/wp-content/uploads/2009/05/kool_aid_man_glass.jpg
http://www.thinkingfinance.net/blog/wp-content/uploads/2009/04/koolaidman.gif
http://tux.crystalxp.net/png/chozo-mj-tux-gandalf-le-gris-1887.png
http://icanhascheezburger.files.wordpress.com/2009/01/funny-pictures-cat-does-not-li
http://midlifeslices.files.wordpress.com/2009/02/explosion-finger.jpg
http://bmorearty.files.wordpress.com/2008/06/test-log-with-titles.png
http://www.adoptedthemovie.com/wp-content/uploads/Picture_005.png
http://comps.fotosearch.com/comp/TBZ/TBZ175/dynamite-magnifying-glass_~DYN014.jpg

Contact info
Available for hire
patrick.huesler@gmail.com
https://www.xing.com/profile/Patrick_Huesler
http://twitter.com/phuesler
http://github.com/phuesler