Running with Ruby

Tag: Rails 4.1

ActiveRecord count vs length vs size and what will happen if you use it the way you shouldn’t

One of the most common and most deadly errors you can make: using length instead of count. You can repeat this multiple times, but you will always find someone who’ll use it the way it shouldn’t be used.

So, first just to make it clear:

#count – collection.count

  • Counts number of elements using SQL query (SELECT COUNT(*) FROM…)
  • #count result is not stored internally during object life cycle, which means, that each time we invoke this method, SQL query is performed again
  • count is really fast comparing to length
2.1.2 :048 > collection = User.all; nil
 => nil
2.1.2 :049 > collection.count
   (0.7ms)  SELECT COUNT(*) FROM `users`
 => 16053
2.1.2 :050 > collection.count
 => 16053

#length – collection.length

  • Returns length of a collecion without performing additional queries… as long as collection is loaded
  • When we have lazy loaded collection, length will load whole colletion into memory and then will return length of it
  • Might use all of your memory when used in a bad way
  • Really fast when having a eagerly loaded collection
2.1.2 :055 > collection = User.all; nil
 => nil
2.1.2 :056 > collection.length
  User Load (122.9ms)  SELECT `users`.* FROM `users`
 => 16053
2.1.2 :057 > collection = User.all; nil
 => nil
2.1.2 :058 > collection.to_a; nil
  User Load (140.9ms)  SELECT `users`.* FROM `users`
 => nil
2.1.2 :059 > collection.length
 => 16053
2.1.2 :060 > collection.length
 => 16053

#size – collection.size

  • Combines abilities of both previous methods;
  • If collection is loaded, will count it’s elements (no additional query)
  • If collection is not loaded, will perform additional query
2.1.2 :034 > collection = User.all; nil
 => nil 
2.1.2 :035 > collection.count
   (0.3ms)  SELECT COUNT(*) FROM `users`
 => 16053 
2.1.2 :036 > collection.count
   (0.3ms)  SELECT COUNT(*) FROM `users`
 => 16053 
2.1.2 :037 > collection.size
   (0.2ms)  SELECT COUNT(*) FROM `users`
 => 16053 
2.1.2 :038 > collection.to_a; nil
  User Load (64.2ms)  SELECT `users`.* FROM `users`
 => nil 
2.1.2 :039 > collection.size
 => 16053 

Why would you even care?

Well it might have a huge impact on your apps performance (and resource consumption). In general if you don’t want to care at all and you want to delegate this responsibility to someone else, use #size. If you want to care, then play with it and understand how it works, otherwise you might end up doing something like this:

print "We have #{User.all.length} users!"

And this is the performance difference on my computer (with only 16k users):

       user     system      total        real
count     0.010000   0.000000   0.010000 (  0.002989)
length    0.730000   0.060000   0.790000 (  0.846671)

Nearly 1 second to perform such simple task. And this could have a serious impact on your web app! Keep that in mind.

Upgrading to Rails 4.1 from Rails 4.0 – Ruby on Rails

Upgrading to Ruby on Rails 4.1 was much easier than moving from 3.2 to 4.0. Maybe because I try to keep all the apps up-2-date, maybe because Rails guys didn’t change much stuff ;) (or maybe both). Either way, lets get through it.

Paperclip – String based terminators are deprecated, please use a lambda

DEPRECATION WARNING: String based terminators are deprecated, please use a lambda. 
(called from has_attached_file at app/config/initializers/paperclip_extensions.rb:22)

Well this one is really simple – just update Paperclip gem:

bundle update paperclip

More about this issue here.

The ability to pass in strings as a class name to set_fixture_class
will be removed

Next deprecation warning:

DEPRECATION WARNING: The ability to pass in strings as a class name to `set_fixture_class`
 will be removed in Rails 4.2. Use the class itself instead. 
(called from block in initialize at gems/activerecord-4.1.0/lib/active_record/fixtures.rb:465)

It you use Rspec you probably won’t see this issue at all (or if you don’t use fixtures). One of my apps unfortunately still does. Solution to this is really simple. Instead of:

set_fixture_class scanlation_categories:  'Scanlation::Category'
set_fixture_class scanlation_chapters:    'Scanlation::Chapter'
set_fixture_class scanlation_pages:       'Scanlation::Page'

use the class name itself (not its string version):

set_fixture_class scanlation_categories:  Scanlation::Category
set_fixture_class scanlation_chapters:    Scanlation::Chapter
set_fixture_class scanlation_pages:       Scanlation::Page

NameError: undefined method `_run_suite’ for class `Test::Unit::Runner’

MiniTest::Unit::TestCase is now Minitest::Test. From /unit/testcase.rb:8:in `<module:Unit>'
rake aborted!
NameError: undefined method `_run_suite' for class `Test::Unit::Runner'
gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
gems/activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
app/test/test_helper.rb:23:in `<top (required)>'

Just get rid of this line from your test/test_helper.rb file:

require 'test/unit'

cannot load such file — polyamorous (LoadError)

in `require': cannot load such file -- polyamorous (LoadError)
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from /ransack/lib/ransack/adapters/active_record/context.rb:3:in `<top (required)>'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from gems/ransack-d51c78f9071f/lib/ransack/adapters/active_record.rb:4:in `<top (required)>'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
from /activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from gems/ransack-d51c78f9071f/lib/ransack.rb:24:in `<top (required)>'
from /bundler-1.5.2/lib/bundler/runtime.rb:76:in `require'
from /bundler-1.5.2/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
from /bundler-1.5.2/lib/bundler/runtime.rb:72:in `each'
from /bundler-1.5.2/lib/bundler/runtime.rb:72:in `block in require'
from /bundler-1.5.2/lib/bundler/runtime.rb:61:in `each'
from /bundler-1.5.2/lib/bundler/runtime.rb:61:in `require'
from /bundler-1.5.2/lib/bundler.rb:131:in `require'
from app/config/application.rb:6:in `<top (required)>'
from app/config/environment.rb:1:in `require'
from app/config/environment.rb:1:in `<top (required)>'
from app/spec/spec_helper.rb:21:in `require'
from app/spec/spec_helper.rb:21:in `<top (required)>'
from app/spec/controllers/portal/announcements_controller_spec.rb:1:in `require'
from app/spec/controllers/portal/announcements_controller_spec.rb:1:in `<top (required)>'
from /rspec-core-3.0.0.beta2/lib/rspec/core/configuration.rb:932:in `load'
from /rspec-core-3.0.0.beta2/lib/rspec/core/configuration.rb:932:in `block in load_spec_files'
from /rspec-core-3.0.0.beta2/lib/rspec/core/configuration.rb:932:in `each'
from /rspec-core-3.0.0.beta2/lib/rspec/core/configuration.rb:932:in `load_spec_files'
from /rspec-core-3.0.0.beta2/lib/rspec/core/command_line.rb:21:in `run'
from /rspec-core-3.0.0.beta2/lib/rspec/core/runner.rb:100:in `run'
from /rspec-core-3.0.0.beta2/lib/rspec/core/runner.rb:31:in `invoke'
from /rspec-core-3.0.0.beta2/exe/rspec:4:in `<top (required)>'
from /home/mencio/.rvm/gems/ruby-2.1.0@senpuu/bin/rspec:23:in `load'
from /home/mencio/.rvm/gems/ruby-2.1.0@senpuu/bin/rspec:23:in `<main>'
from /home/mencio/.rvm/gems/ruby-2.1.0@senpuu/bin/ruby_executable_hooks:15:in `eval'
from /home/mencio/.rvm/gems/ruby-2.1.0@senpuu/bin/ruby_executable_hooks:15:in `<main>'

To remove this issue, upgrade your Ransack, MetaSearch and Squeel gems to newest versions and/or add this to your Gemfile:

gem 'polyamorous', github: 'activerecord-hackery/polyamorous'

ActionView::Template::Error:
 undefined method `reverse!' for #<ActiveRecord::Relation []>
Shared Example Group: "has valid single" 
called from ./spec/controllers/episodes_controller_spec.rb:13
./lib/system/active_record/nearable.rb:54:in `near'
./app/views/portal/episodes/show.html.haml:43:in `block in _app_views_episodes_show_html_haml'
./app/views/portal/episodes/show.html.haml:1:in `_app_views_episodes_show_html_haml'
./app/controllers/application_controller.rb:68:in `respond_with'
./app/controllers/portal/base_controller.rb:64:in `respond_with'
./app/controllers/portal/episodes_controller.rb:13:in `show'
./spec/support/macros/controllers/actions.rb:80:in `block (4 levels) in <module:Actions>'

With ActiveRecord 4.1, you can’t call reverse! directly on ActiveRecord::Relation. Example:

# This will throw an error
@articles = Article.limit(10).order('created_at DESC').reverse!

Instead you have to cast ActiveRecord::Relation to an array:

@articles = Article.limit(10).order('created_at DESC').to_a.reverse!

Keep in mind, that casting with to_a will deprive you from all benefits of lazy loading with Rails relations so use it carefully. But on the other hand, reverse! on ActiveRecord::Relation did the same, so if you used it and it was ok, than feel free ;)

Undefined method `graft’ for class ActiveRecord::Associations::JoinDependency

gems/activesupport-4.1.0/lib/active_support/core_ext/module/aliasing.rb:32:in `alias_method': 
undefined method `graft' for class `ActiveRecord::Associations::JoinDependency' (NameError)
activesupport-4.1.0/lib/active_support/core_ext/module/aliasing.rb:32:in `alias_method_chain'
from gems/polyamorous-0.6.4/lib/polyamorous/join_dependency.rb:7:in `block in included'
from gems/polyamorous-0.6.4/lib/polyamorous/join_dependency.rb:5:in `class_eval'
from gems/polyamorous-0.6.4/lib/polyamorous/join_dependency.rb:5:in `included'
from gems/polyamorous-0.6.4/lib/polyamorous.rb:20:in `include'
from gems/polyamorous-0.6.4/lib/polyamorous.rb:20:in `<top (required)>'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from gems/ransack/lib/ransack/adapters/active_record/context.rb:3:in `<top (required)>'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `block in require'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:232:in `load_dependency'
from gems/activesupport-4.1.0/lib/active_support/dependencies.rb:247:in `require'

Just install the newest version of ransack gem.

gem 'ransack', github: 'ernie/ransack'

Scoped order and limit are ignored, it’s forced to be batch order and batch size

This is quite logical – you can’t have some of orders when batching (for example a RAND()). That’s why ActiveRecord is ignoring it. Just keep that in mind ;)

Example:

Token.2.1.0 :001 > Token.order('RAND()').find_each{}
W, [2014-05-09T11:26:46.601539 #11569]  WARN -- : 
Scoped order and limit are ignored, it's forced to be batch order and batch size
D, [2014-05-09T11:26:46.698603 #11569] DEBUG -- :   
Token Load (3.7ms)  SELECT  "accounts".* FROM "accounts"   ORDER BY "accounts"."id" ASC LIMIT 1000

Other issues

Well to be honest I didn’t have any more issues. I’ve decided to remove Squeel gem from all of my projects, since it is not currently maintained. Thanks to that I’ve finally got rid of this irritating deprecation warning:

DEPRECATION WARNING: Core extensions are deprecated and will be removed in Squeel 2.0.
 (called from /app/config/initializers/squeel.rb:2:in `block in <top (required)>')

Summary

Rails 4.1 is not a big step, although it is a required one if you want to upgrade to 4.2 in the future. If you have decent test coverage level, you should not have big issues with this upgrade.

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