Enumerable#index _by For Better Hash Building In Rails

Ruby hashes are awesome! They allow us to efficiently access data by a key. A common task in data processing is to build hashes using Enumerable#each_with_object so we can quickly access related data during processing steps

In this example, we need to process a large amount of photo data by invoking process_photo, which needs a photo from our data and an instance from our Photographer model

# Bad, we will perform photo_data.count database requests
photo_data.each do |photo|
  process_photo(photo, Photographer.find_by(key: photo.key))
end

# Better, we perform a single database request
all_keys = photo_data.map(&:key).uniq
photographers_by_key = Photographer
                       .where(key: all_keys)
                       .each_with_index({}) do |photographer, accumulator|
  accumulator[photographer.key] = photographer
end
photo_data.each do |photo|
  process_photo(photo, photographers_by_key[photo.key])
end

Thanks to a recent addition to rubocop-rails, I learned that we can use Rails’ ActiveSupport method Enumerable#index_by to make this manipulation even easier

# Even Better, Enumerable#index_by simplifies the construction of our hash
all_keys = photo_data.map(&:key).uniq
photographers_by_key = Photographer.where(key: all_keys).index_by(&:key)
photo_data.each do |photo|
  process_photo(photo, photographers_by_key[photo.key])
end

TLDR: use Enumerable#index_by over Enumerable#each_with_object

# Good
photographers_by_key = Photographer
                       .where(key: all_keys)
                       .each_with_index({}) do |photographer, accumulator|
accumulator[photographer.key] = photographer
end

# Better
photographers_by_key = Photographer.where(key: all_keys).index_by(&:key)

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May 18, 2020

Enumerable#index _by For Better Hash Building In Rails

Chris White

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Back to blog overview

May 18, 2020

Enumerable#index _by For Better Hash Building In Rails

Chris White

&

&

Let's Chat

conference for builders

Trever Yarrish

Zeal in 2023: A Year of Empowering Innovation and Building Connections

Jesse House

Managing Secrets: credentials.yml vs. Environment Variables in Ruby on Rails

Jesse House

How to add TailwindCSS to Ruby on Rails

conference
for builders