gistfile1.rb

#!/usr/bin/env ruby

require "csv"
require "yaml"

product_id   = ARGV[0].to_i
path_to_csv  = ARGV[1]
path_to_yaml = 'hoge.yaml'

hash = {}
hash["products"] = {
  product_id => {
    "product_id"    => product_id
  }
}

csv = CSV.read(path_to_csv, headers: :first_row)
csv.each do |row|
  user_id    = row["user_id"].to_i
  email      = row["email"]
  quantiy    = row["quantiy"].to_i
  users_hash = hash["products"][product_id]["elected_users"]
  if users_hash
    user_hash  = {
      "user_id"  => user_id,
      "quantity" => quantiy
    }
    hash["products"][product_id]["elected_users"][user_id] = user_hash
  else
    user_hash  = {
      user_id => {
        "user_id"  => user_id,
        "quantity" => quantiy
      }
    }
    hash["products"][product_id]["elected_users"] = user_hash
  end
  p hash
end

File.open(path_to_yaml, "w") { |f| f.write(hash.to_yaml) }