Make selecting or rejecting items from an Ohm collection faster by getting only selected fields.

example.rb

require 'benchmark'

User.all.size
#=> 30000

# BEFORE
Benchmark.measure { User.all.select { |user| user.firstname =~ /Erol/ } }
#=>  66.630000   1.070000  67.700000 ( 67.868905)

# AFTER
Benchmark.measure { User.all.select(:firstname) { |firstname| firstname =~ /Erol/ } }
#=>   2.080000   0.020000   2.100000 (  2.231251) 

ohm.rb

module Ohm::Collection
  def select(*atts)
    return Enumerator.new(self, :select, *atts) unless block_given?
    return super if atts.empty?

    ids = key.sort(by: '#', get: ['#'] + atts.map { |att| "#{model.key}:*->#{att}" })
      .select { |values| yield *values.slice(1..-1) }
      .map { |values| values[0] }
    fetch ids
  end

  def reject(*atts)
    return Enumerator.new(self, :select, *atts) unless block_given?
    return super if atts.empty?

    ids = key.sort(by: '#', get: ['#'] + atts.map { |att| "#{model.key}:*->#{att}" })
      .reject { |values| yield *values.slice(1..-1) }
      .map { |values| values[0] }
    fetch ids
  end
end