# app/models/concerns/users/csv_conversion.rb

class User
  module CsvConversion
    extend ActiveSupport::Concern

    included do
      def to_csv(options = {})
        CSV.generate(options) do |csv|
          csv << %w[id username email]
          all.each do |user|
            csv << [user.id, user.username, user.email]
          end
        end
      end
    end
  end
end

# app/models/user.rb
include CsvConversion

# Use modules for more complex and widely used logic

# Using modules/plugins
# in lib/plugins/sponsorable.rb
module Sponsorable
  extend ActiveSupport::Concern
  module ClassMethods
    def acts_as_sponsorable(configuration = {})
      # Do your logic and define the needed associations ex: product has many sponsors,
      # A sponsor may sponsor many products , yes a polymorphic association
    end

    # If you forgot this line all instances of ActiveRecord::Base will have these methods !
    include InstanceMethods
  end

  module InstanceMethods
   def sponsors_count
   end
  end
end

ActiveRecord::Base.send(:include, Sponsorable)

# config/initializers/extension.rb
require 'sponsorable'

# app/models/product.rb
acts_as_sponsorable({ max_count: 5, sponsors_type: 'exclusive' })

# Use modules/plugins for shared complex logic and use concerns for model related simple logic