> ## Documentation Index
> Fetch the complete documentation index at: https://spreecommerce.org/docs/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](/developer/customization/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.

<Info>
  This tutorial uses decorators for extending Spree models. For extensions that need to react to events (sync with external services, send notifications, etc.), consider using [Events subscribers](/developer/core-concepts/events) instead - they're easier to test and maintain. See [Customization Quickstart](/developer/customization/quickstart) for guidance on choosing the right approach.
</Info>

## 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.

Install the Spree Extension CLI by running:

```bash theme={"theme":"night-owl"}
gem install spree_extension
```

Run the following command from a directory of your choice outside of our Spree application:

```bash theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
bin/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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
# context: Your Spree store's app root (i.e. Rails.root); not the extension's root path.
bin/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\_storefront 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 theme={"theme":"night-owl"}
mkdir -p app/controllers/spree_simple_sales
```

Next, create a new file in the directory we just created called `home_controller_decorator.rb` and add the following content to it:

```ruby theme={"theme":"night-owl"}
module SpreeSimpleSales
  module HomeControllerDecorator
    def sale
      @products = Spree::Product.joins(:variants_including_master).where.not(sale_price: nil).distinct
    end
  end
end

Spree::HomeController.prepend SpreeSimpleSales::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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
bin/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 theme={"theme":"night-owl"}
> product = Spree::Product.first
=> #<Spree::Product id: 107377505, name: "Spree Bag", description: "Lorem ipsum dolor sit amet, consectetuer adipiscing...", available_on: "2013-02-13 18:30:16", deleted_at: nil, permalink: "spree-bag", meta_description: nil, meta_keywords: nil, tax_category_id: 25484906, shipping_category_id: nil, count_on_hand: 10, created_at: "2013-02-13 18:30:16", updated_at: "2013-02-13 18:30:16", on_demand: false>

> variant = product.master
=> #<Spree::Variant id: 833839126, sku: "SPR-00012", weight: nil, height: nil, width: nil, depth: nil, deleted_at: nil, is_master: true, product_id: 107377505, count_on_hand: 10, cost_price: #<BigDecimal:7f8dda5eebf0,'0.21E2',9(36)>, 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.

Next, create the file `app/models/spree_simple_sales/variant_decorator.rb` and add the following content to it:

```ruby theme={"theme":"night-owl"}
module SpreeSimpleSales
  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

Spree::Variant.prepend SpreeSimpleSales::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 theme={"theme":"night-owl"}
bundle exec rake test_app
```

After this command completes, you should be able to run `rspec` and see the following output:

```bash theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
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 theme={"theme":"night-owl"}
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.

## Alternative Approaches

While this tutorial uses decorators to extend Spree's core behavior, modern Spree provides additional patterns that may be more appropriate depending on your use case:

| Use Case                                       | Recommended Approach                                            |
| ---------------------------------------------- | --------------------------------------------------------------- |
| Structural changes (associations, validations) | Decorators (as shown in this tutorial)                          |
| React to model changes                         | [Events subscribers](/developer/core-concepts/events)           |
| External service integration                   | [Webhooks](/developer/core-concepts/webhooks)                   |
| Replace core services                          | [Dependencies injection](/developer/customization/dependencies) |
| Add admin UI elements                          | [Admin Partials](/developer/admin/extending-ui)                 |
| Add admin menu items                           | [Admin Navigation](/developer/admin/navigation)                 |

For example, if your extension needs to sync data with an external service when products are updated, use an Events subscriber instead of a decorator callback:

```ruby app/subscribers/my_extension/product_sync_subscriber.rb theme={"theme":"night-owl"}
module MyExtension
  class ProductSyncSubscriber < Spree::Subscriber
    subscribes_to 'product.updated'

    def handle(event)
      product = Spree::Product.find_by(id: event.payload['id'])
      return unless product

      ExternalService.sync(product)
    end
  end
end
```

## Related Documentation

* [Events](/developer/core-concepts/events) - Subscribe to Spree events
* [Webhooks](/developer/core-concepts/webhooks) - HTTP callbacks for external integrations
* [Dependencies](/developer/customization/dependencies) - Replace core services
* [Customization Quickstart](/developer/customization/quickstart) - Choose the right approach