> ## Documentation Index > Fetch the complete documentation index at: https://spreecommerce-documentation-update-4-5-to-4-6.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # Creating an Extension > Learn how to create a Spree extension. ## Overview [Spree Extensions](/extending-spree/3rd-party-extensions) are a way to add new functionality to your Spree store. They are a great way to extend the functionality of Spree and add new features. You can share them with Spree community on Github so anyone can use them, contribute back and share your improvements. ## Getting Started Let's build a simple extension. Suppose we want the ability to mark certain products as being on sale. We'd like to be able to set a sale price on a product and show products that are on sale on a separate products page. This is a great example of how an extension can be used to build on the solid Spree foundation. Before we start, let's make sure we have Spree CLI installed by running: ```bash gem install spree_cmd ``` So let's start by generating the extension. Run the following command from a directory of your choice outside of our Spree application: ```bash spree extension simple_sales ``` This creates a `spree_simple_sales` directory with several additional files and directories. After generating the extension make sure you change to its directory: ```bash cd spree_simple_sales ``` ## Adding a Sale Price to Variants The first thing we need to do is create a migration that adds a sale\_price column to [variants](/developer/core-concepts/products#variants). We can do this with the following command: ```bash bundle exec rails g migration add_sale_price_to_spree_variants sale_price:decimal ``` Because we are dealing with prices, we need to now edit the generated migration to ensure the correct precision and scale. Edit the file `db/migrate/XXXXXXXXXXX_add_sale_price_to_spree_variants.rb` so that it contains the following: ```ruby class AddSalePriceToSpreeVariants < ActiveRecord::Migration[7.1] def change add_column :spree_variants, :sale_price, :decimal, precision: 8, scale: 2 end end ``` ## Adding Our Extension to the Spree Application Before we continue development of our extension, let's add it to the Spree application. Within the `my_store` application directory, add the following line to the bottom of our `Gemfile`: ```ruby gem 'spree_simple_sales', path: '../spree_simple_sales' ``` You may have to adjust the path somewhat depending on where you created the extension. You want this to be the path relative to the location of the `my_store` application. Once you have added the gem, it's time to bundle: ```bash bundle install ``` Finally, let's run the `spree_simple_sales` install generator to copy over the migration we just created (answer **yes** if prompted to run migrations): ```bash # context: Your Spree store's app root (i.e. Rails.root); not the extension's root path. bundle exec rails g spree_simple_sales:install ``` ## Adding a Controller Action to HomeController Now we need to extend `Spree::HomeController` and add an action that selects "on sale" products. Note for the sake of this example that \`Spree::HomeController\` is only included in spree\_frontend so you need to make it a dependency on your extensions \*.gemspec file. Make sure you are in the `spree_simple_sales` root directory and run the following command to create the directory structure for our controller decorator: ```bash mkdir -p app/controllers/spree_simple_sales/spree ``` Next, create a new file in the directory we just created called `home_controller_decorator.rb` and add the following content to it: ```ruby module SpreeSimpleSales module Spree module HomeControllerDecorator def sale @products = Spree::Product.joins(:variants_including_master).where('spree_variants.sale_price is not null').distinct end end end end Spree::HomeController.prepend SpreeSimpleSalesSpree::HomeControllerDecorator ``` This will select just the products that have a variant with a `sale_price` set. We also need to add a route to this action in our `config/routes.rb` file. Let's do this now. Update the routes file to contain the following: ```ruby Spree::Core::Engine.routes.draw do get "/sale" => "home#sale" end ``` ## Viewing On Sale Products ### Setting the Sale Price for a Variant Now that our variants have the attribute `sale_price` available to them, let's update the sample data so we have at least one product that is on sale in our application. We will need to do this in the rails console for the time being, as we have no admin interface to set sale prices for variants. So, in order to do this, first open up the rails console: ```bash bundle exec rails c ``` Now, follow the steps I take in selecting a product and updating its master variant to have a sale price. Note, you may not be editing the exact same product as I am, but this is not important. We just need one "on sale" product to display on the sales page. ```ruby > product = Spree::Product.first => # > variant = product.master => #, position: nil, lock_version: 0, on_demand: false, cost_currency: nil, sale_price: nil> > variant.sale_price = 8.00 => 8.0 > variant.save => true ``` ## Decorating Variants Let's fix our extension so that it uses the `sale_price` when it is present. First, create the required directory structure for our new decorator: ```bash mkdir -p app/models/spree_simple_sales/spree ``` Next, create the file `app/models/spree_simple_sales/spree/variant_decorator.rb` and add the following content to it: ```ruby module SpreeSimpleSales module Spree module VariantDecorator def price_in(currency) return super unless sale_price.present? Spree::Price.new(variant_id: self.id, amount: self.sale_price, currency: currency) end end end end Spree::Variant.prepend SpreeSimpleSalesSpree::VariantDecorator ``` If there is a `sale_price` present on the product's master variant, we return that price. Otherwise, we call the original implementation of `price_in` (using `return super`). ## Testing Our Decorator It's always a good idea to test your code. We should be extra careful to write tests for our Variant decorator since we are modifying core Spree functionality. Let's write a couple of simple unit tests for `variant_decorator.rb` ### Generating the Test App An extension is not a full Rails application, so we need something to test our extension against. By running the Spree `test_app` rake task, we can generate a barebones Spree application within our `spec` directory to run our tests against. We can do this with the following command from the root directory of our extension: ```bash bundle exec rake test_app ``` After this command completes, you should be able to run `rspec` and see the following output: ```bash No examples found. Finished in 0.00005 seconds 0 examples, 0 failures ``` Great! We're ready to start adding some tests. Let's replicate the extension's directory structure in our spec directory by running the following command ```bash mkdir -p spec/models/spree ``` Now, let's create a new file in this directory called `variant_decorator_spec.rb` and add the following tests to it: ```ruby require 'spec_helper' describe Spree::Variant do describe "#price_in" do it "returns the sale price if it is present" do variant = create(:variant, sale_price: 8.00) expected = Spree::Price.new(variant_id: variant.id, currency: "USD", amount: variant.sale_price) result = variant.price_in("USD") expect(result.variant_id).to eq(expected.variant_id) expect(result.amount.to_f).to eq(expected.amount.to_f) expect(result.currency).to eq(expected.currency) end it "returns the normal price if it is not on sale" do variant = create(:variant, price: 15.00) expected = Spree::Price.new(variant_id: variant.id, currency: "USD", amount: variant.price) result = variant.price_in("USD") expect(result.variant_id).to eq(expected.variant_id) expect(result.amount.to_f).to eq(expected.amount.to_f) expect(result.currency).to eq(expected.currency) end end end ``` These specs test that the `price_in` method we overrode in our `VariantDecorator` returns the correct price both when the sale price is present and when it is not. ## Summary In this tutorial, you learned how to both install extensions and create your own. A lot of core Spree development concepts were covered and you gained exposure to some of the Spree internals.