Fixing ActiveRecord_Relation#pluck vs map

1 - benchmark.rb

require 'benchmark/ips'

collection = User.limit(1)

module ActiveRecord
  module Calculations
    alias_method :old_pluck, :pluck

    # original: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/calculations.rb#L184-L201
    def pluck(*column_names)
      if loaded?
        # If only one column is given, and it's an attribute, just call the attribute method
        if column_names.length == 1 && @klass.has_attribute?(column_names.first)
          return records.map{ |r| r._read_attribute(column_names.first) }
        elsif (column_names.map(&:to_s) - @klass.attribute_names - @klass.attribute_aliases.keys).empty?
          # otherwise fallback to previous behaviour
          return records.pluck(*column_names)
        end
      end

      if has_include?(column_names.first)
        relation = apply_join_dependency
        relation.pluck(*column_names)
      else
        enforce_raw_sql_whitelist(column_names)
        relation = spawn
        relation.select_values = column_names.map { |cn|
          @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
        }
        result = skip_query_cache_if_necessary { klass.connection.select_all(relation.arel, nil) }
        result.cast_values(klass.attribute_types)
      end
    end
  end
end

Benchmark.ips do |x|
  x.report('map') { collection.map(&:first_name) }
  x.report('old_pluck') { collection.old_pluck(:first_name) }
  x.report('pluck') { collection.pluck(:first_name) }
  x.compare!
end

gistfile1.txt

Warming up --------------------------------------
                 map    56.596k i/100ms
           old_pluck    11.714k i/100ms
               pluck    65.071k i/100ms
Calculating -------------------------------------
                 map    752.586k (± 4.9%) i/s -      3.792M in   5.051037s
           old_pluck    121.499k (± 3.8%) i/s -    609.128k in   5.020716s
               pluck    781.728k (± 5.8%) i/s -      3.904M in   5.013820s

Comparison:
               pluck:   781728.0 i/s
                 map:   752586.3 i/s - same-ish: difference falls within error
           old_pluck:   121499.3 i/s - 6.43x  slower