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Overview

All of Spree’s models, controllers, services,helpers, etc can easily be extended or overridden to meet your exact requirements using standard Ruby idioms. Standard practice for including such changes in your application or extension is to create a file within the relevant app/models/spree or app/controllers/spree directory with the original class name with _decorator appended, eg. for the Spree::Product model, create a file called app/models/spree/product_decorator.rb.

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Decorating Models

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Changing behavior of existing methods

Changing the behavior of existing methods is the most common use case for decorators. Let’s say you want to change the behavior of the available? in the Product model.
  1. Let’s start with creating a file in app/models/spree/product_decorator.rb 
    mkdir -p app/models/spree && touch app/models/spree/product_decorator.rb
  2. And add the following code to the file: 
    module Spree
  module ProductDecorator
    def available?
      # your code here
    end
  end

  Product.prepend(ProductDecorator)
end

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Adding a new method to the model

You can also add a new method to the model. Or even add custom validations, callbacks, etc. 
module Spree
  module ProductDecorator
    def self.prepended(base)
      base.before_validation :strip_whitespaces
    end

    protected

    def strip_whitespaces
      self.name = name.strip
    end
  end

  Product.prepend(ProductDecorator)
end

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Adding new associations to Spree models

Assume you want to add a new model called Video associated to Spree::Product. Let’s start with creating a database migration: 
bin/rails g migration CreateVideos url:string product:references
bin/rails db:migrate
In the model code add the association to Spree::Product: 
class Video < ApplicationRecord
  belongs_to :product, class_name: 'Spree::Product'

  has_one_attached :video_file
end
Finally add the association in ProductDecorator in app/models/spree/product_decorator.rb: 
module Spree
  module ProductDecorator
    def self.prepended(base)
      base.has_many :videos, class_name: 'Video', foreign_key: 'product_id', dependent: :destroy
    end
  end

  Product.prepend(ProductDecorator)
end

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Decorating Controllers

Adding a custom action to the ProductsController
  1. Create a file in app/controllers/spree/products_controller_decorator.rb 
    mkdir -p app/controllers/spree && touch app/controllers/spree/products_controller_decorator.rb
  2. Add the following code: 
    module Spree
  module ProductsControllerDecorator
    def some_action
      # your code here
    end
  end

  ProductsController.prepend(ProductsControllerDecorator)
end
The exact same format can be used to redefine an existing method.

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Accessing Product Data

If you extend the Products controller with a new method, you may very well want to access product data in that method. You can do so by using the :load_data before_action. 
module Spree
  module ProductsControllerDecorator
    def self.prepended(base)
      base.before_action :load_data, only: :some_action
    end

    def some_action
      # your code here
    end
  end

  ProductsController.prepend(ProductsControllerDecorator)
end
:load_data will use params[:id] to lookup the product by its permalink.