Separate CSV in slices

separate.rb

require "csv"

current = 0

read_to_array = CSV.read("all.csv", :headers => true)
unique_user_ids = read_to_array.uniq { |u| u["user_id"] }

unique_user_ids.each_slice(10000) do |rows|
  CSV.open("slice-#{current}.csv", "w") do |csv|
    rows.each do |row|
      csv << row

      print "."
      current += 1
    end
  end
end