Concern

1 - rails new concern
rails generate model Product name company:text
rails generate model Category name:text
rails generate model Tag name:text product_id category_id:integer

2 - Arquivo seed
require 'faker'
produtos = (1..100).to_a
categorias = (1..5).to_a

100.times do
    Product.create(name: Faker::Name.name, company: Faker::Company.name)
end

5.times do
    Category.create(name: Faker::Commerce.department)
end

tagname = ['esporte', 'cama', 'mesa', 'banho', 'eletronico', 'telefonia', 'ferramentas', 'livros' ]

tagname.each {|tag|
    Tag.create(name: tagname.sample, product_id: produtos.sample, category_id: categorias.sample )
}


3 e 4 - class Product < ActiveRecord::Base
    has_many :tags

    def list_tags
        tags.map(&:name).join(',')
    end
end

class Category < ActiveRecord::Base
    has_many :tags

    def list_tags
        tags.map(&:name).join(',')
    end
end

5- module Concerns::Maps
  extend ActiveSupport::Concern

def list_tags
    tags.map(&:name).join(',')
end
end