Ruby Archives - Page 45 of 97

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.

If you don't have multiple cores and/or you have a small VPN, you may end up with a huge slow down of your web app, when rake tasks are executed. This can be a big issue especially when you use something like whenever to perform periodic tasks. Luckily there's a nice program:

nice is a program found on Unix and Unix-like operating systems such as Linux. It directly maps to a kernel call of the same name. nice is used to invoke a utility or shell script with a particular priority, thus giving the process more or less CPU time than other processes. A niceness of −20 is the highest priority and 19 or 20 is the lowest priority. The default niceness for processes is inherited from its parent process and is usually 0.

With nice you can tell your Linux kernel to give your rake tasks the lowest priority possible. Thanks to that, you can make sure, that any task with higher priority (like web UI server) won't get stuck because of the background rake task.

Here's how you can use it:

RAILS_ENV=:environment nice -n 19 bundle exec rake :your_task

If you use a gem like whenever, you can easily integrate nice with it:

job_type :runner, "cd :path && nice -n 19 script/rails runner -e :environment ':task' :output"

every 30.minutes do
  runner 'Worker.new.perform'
end

or for a rake task with whenever, you can use this:

job_type :runner, "cd :path && :environment_variable=:environment nice -19 bundle exec rake :task --silent :output"

every 15.minutes do
  runner 'your_task'
end

Note: Of course this will help, when CPU is your bottleneck. Keep in mind that you're rake task might slow down your website because of many more reasons (IO, DB, etc).

Tag: Ruby

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

#count - collection.count

#length - collection.length

#size - collection.size

Why would you even care?

Ruby & Rails: Making sure rake task won’t slow the site down