在 ./db/seeds/my_module.rb:

module MyModule
  puts "In my_module.rb"
  # add code here
end

在./db/seeds.rb中:

require File.expand_path('../seeds/my_module', __FILE__) # the ../ just removes `seeds.rb` filename from the path which is given by __FILE__

p "In seeds.rb"
# add code here