how.rb

# Current

def initialize(resource = nil)
  @resource = resource
end

private

def resource
  @resource ||= ProductResource.new(connection: ExternalAdapter.instance.connection)
end

# Scenario 1

def initialize(resource = ProductResource.new(connection: ExternalAdapter.instance.connection))
  @resource = resource
end

# Scenario 2

def initialize(resource = ProductResource.build)
  @resource = resource
end

class ProductResource
  def self.build(adapter = ExternalAdapter.instance.connection)
    new(connection: adapter)
  end
end