Stamp.rb

class Stamp < ActiveRecord::Base
    ####途中省略####
  if Rails.env.production?
    S3_CREDENTIALS = {access_key_id: ENV['S3_ACCESS_KEY_ID'], secret_access_key: ENV['S3_SECRET_KEY'], bucket: "hoge-bucket"}
  end
  if Rails.env.production?
    has_attached_file :stamp, storage: :s3, s3_credentials: S3_CREDENTIALS, 
                              styles: {stamps_s: "100x100&gt;", stamps: "350x350&gt;"}, url: ":s3_domain_url", path: "stamps/:style/:filename"
  else
    has_attached_file :stamp, url: "/:style/:img_dir_num/:filename", styles: {stamps_s: "100x100&gt;", stamps: "350x350&gt;"}
  end
end