Collect an enumerable into sets, grouped by the result of a block. Useful, for example, for grouping records by date.

e.g.

  latest_transcripts.group_by(&:day).each do |day, transcripts|
    p "#{day} -> #{transcripts.map(&:class) * ', '}"
  end
  "2006-03-01 -> Transcript"
  "2006-02-28 -> Transcript"
  "2006-02-27 -> Transcript, Transcript"
  "2006-02-26 -> Transcript, Transcript"
  "2006-02-25 -> Transcript"
  "2006-02-24 -> Transcript, Transcript"
  "2006-02-23 -> Transcript"

[Source]

    # File vendor/rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 17
17:   def group_by
18:     inject({}) do |groups, element|
19:       (groups[yield(element)] ||= []) << element
20:       groups
21:     end
22:   end

Convert an enumerable to a hash. Examples:

  people.index_by(&:login)
    => { "nextangle" => <Person ...>, "chade-" => <Person ...>, ...}
  people.index_by { |person| "#{person.first_name} #{person.last_name}" }
    => { "Chade- Fowlersburg-e" => <Person ...>, "David Heinemeier Hansson" => <Person ...>, ...}

[Source]

    # File vendor/rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 56
56:   def index_by
57:     inject({}) do |accum, elem|
58:       accum[yield(elem)] = elem
59:       accum
60:     end
61:   end

Calculates a sum from the elements. Examples:

 payments.sum { |p| p.price * p.tax_rate }
 payments.sum(&:price)

This is instead of payments.inject { |sum, p| sum + p.price }

Also calculates sums without the use of a block:

  [5, 15, 10].sum # => 30

The default identity (sum of an empty list) is zero. However, you can override this default:

[].sum(Payment.new(0)) { |i| i.amount } # => Payment.new(0)

[Source]

    # File vendor/rails/activesupport/lib/active_support/core_ext/enumerable.rb, line 39
39:   def sum(identity = 0, &block)
40:     return identity unless size > 0
41: 
42:     if block_given?
43:       map(&block).sum
44:     else
45:       inject { |sum, element| sum + element }
46:     end
47:   end