Posted about 1 year ago. Visible to the public.

Custom Ruby method Enumerable#count_by (use for quick statistics)

I frequently find myself needing a combination of group_by, count and sort for quick statistics. Here's a method on Enumerable that combines the three:

module Enumerable def count_by(&block) list = group_by(&block) .map{ |key, items| [key, items.count] } .sort_by(&:last) Hash[list] end end

The new method returns a Hash of key => count pairs.

Usage examples

Number of email addresses by domain:

> User.all.count_by { |user| /^.*@/, '' } => { ""=>2, ..., ""=>63, ""=>128, ""=>153}


  • Number of new users per day: User.all.count_by { |user| user.created_at.to_date }
  • Number of articles per brand: Article.all.count_by &:brand

Note that the last simple example can also be achieved with Rails internals: This translates to SQL, so it executes fast. However, grouping is restricted to columns (attributes). Using #count_by gives you the full flexibility of Ruby.

More tools

If you need further options, here's a "toolbox" of chainable method invocations:

list .group_by{ |item| } # Group .map{ |key, items| [key, items.count] } # Count .select{ |key, count| count > 10 } # Filter .sort_by(&:last) # Sort ASC .sort_by{ |key, count| -count } # Sort DESC .each{ |key, count| puts "#{count.to_s.rjust(6)} #{key}" } # Print

Wrap the result in Hash[...] to turn a list-of-pairs into a Hash.

Does your version of Ruby on Rails still receive security updates?
Rails LTS provides security patches for old versions of Ruby on Rails (3.2 and 2.3).

Owner of this card:

Dominik Schöler
Last edit:
about 1 year ago
by Dominik Schöler
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Dominik Schöler to makandra dev
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