​
Zeitwerk compatibility

Zeitwerk is the new default code autoloader in Rails 6. This doesn’t work well with the old approach to decorators files that name ends with decorator.rb, eg. app/models/spree/order_decorator.rb using class_eval. To fix this we need to convert all class_eval decorators to modules and use Module.prepend. Also we need to name them properly according to Zeitwerk naming rules Example of an old decorator: app/models/spree/order_decorator.rb 
Spree::Order.class_eval do
  has_many :new_custom_model

  def some_method
     # ...
  end
end
the same decorator in the new notation: app/models/your_extension_name/order_decorator.rb 
module YourExtensionName::OrderDecorator
  def self.prepended(base)
    base.has_many :new_custom_model
  end

  def some_method
    # ...
  end
end

Spree::Order.prepend(YourExtensionName::OrderDecorator)

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Fixing Deface Overrides

Please remember to prepare versioned overrides for both Spree 3.x and 4.x, eg. https://github.com/spree-contrib/spree_static_content/commit/e4b9e4900024235158d0ec1a48a100b4732348ef Spree 4 uses Bootstrap 4 and many partials and HTML structure changed compared to Spree 3.x. Also - remember to add deface gem to gemspec as deface itself was removed as a dependency of Spree. eg. https://github.com/spree/spree_auth_devise/commit/d729689ca87d8586e541ffcc865ef1e0a5a79fe4

​
Migrate to Spree Dev Tools

Replace all development dependencies with: 
s.add_development_dependency 'spree_dev_tools'
Replace spec_helper.rb contents with: https://github.com/spree/spree/blob/777a284b4c70e69d32a05ffa61bbe3905d8f1297/cmd/lib/spree_cmd/templates/extension/spec/spec_helper.rb Example migrations: