```ruby
# ------ Examples -------------------------------------------------------------
#
#     et = Etsource::Dataset.new('nl')
#     et.import # => Qernel::Dataset for country 'nl'
#

module Etsource
  class Dataset
    attr_reader :country

    def initialize(country)
      # DEBT: @etsource is only used for the base_dir, can be solved better.
      @etsource = Etsource::Base.instance
      @country  = country
    end

    # Importing dataset and convert into the Qernel::Dataset format.
    # The yml file is a flat (no nested key => values) hash. We move it to a nested hash
    # and also have to convert the keys into a numeric using a hashing function (FNV 1a),
    # the additional nesting of the hash, and hashing ids as strings are mostly for
    # performance reasons.
    #
    def import
      ::Etsource::Dataset::Import.new(country).import
    end

    def self.region_codes
      @region_codes ||= Atlas::Dataset.all
        .select { |dataset| dataset.enabled[:etengine] }
        .map    { |dataset| dataset.key.to_s }
    end
  end
end
```

Expire region code cache when importing ETSource
user
```ruby
# ------ Examples -------------------------------------------------------------
#
#     et = Etsource::Dataset.new('nl')
#     et.import # => Qernel::Dataset for country 'nl'
#

module Etsource
  class Dataset
    attr_reader :country

    def initialize(country)
      # DEBT: @etsource is only used for the base_dir, can be solved better.
      @etsource = Etsource::Base.instance
      @country  = country
    end

    # Importing dataset and convert into the Qernel::Dataset format.
    # The yml file is a flat (no nested key => values) hash. We move it to a nested hash
    # and also have to convert the keys into a numeric using a hashing function (FNV 1a),
    # the additional nesting of the hash, and hashing ids as strings are mostly for
    # performance reasons.
    #
    def import
      ::Etsource::Dataset::Import.new(country).import
    end

    def self.region_codes
      NastyCache.instance.fetch('region_codes') do
        Atlas::Dataset.all
          .select { |dataset| dataset.enabled[:etengine] }
          .map    { |dataset| dataset.key.to_s }
      end
    end
  end
end
```
assistant