Naming is hard

group_by_caching.rb

module Models
  module GroupByCaching
    extend ActiveSupport::Concern

    included do
      CACHE_KEY_PREFIX = "#{self.name.collectionize}_grouped_by"
    end


    module ClassMethods
      # Loads all documents, groups them by the given key and puts them in the cache if there is none yet.
      # @param [Symbol, String] group_by_key the key that will determine the values of the returned hash
      # @param [Hash] caching_config is passed to the concrete ActiveSupport::Cache::Store. See the latter for a detailed description
      # @option caching_config [Numeric] :expires_in sets an expiration time in seconds for the cache entry when it will be automatically removed from the cache.
      # @option caching_config [Numeric] :race_condition_ttl sets the number of seconds that an expired entry can be reused while a new value is being regenerated.
      # @return [Hash] with the keys being the results for the given key, mapped to the loaded documents having that value
      def cache_all_grouped_by(group_by_key, caching_config)
        caching_config.assert_valid_keys(ActiveSupport::Cache::UNIVERSAL_OPTIONS)
        cache_key = get_cache_key(group_by_key)
        Rails.cache.fetch(cache_key, caching_config) do
          all.group_by(&group_by_key)
        end
      end


      # @param [Symbol, String] field of the document that is used to get a cached set
      # @param [Object] value for #field that is used to retrieve the documents from the cache
      # @return [Array<Mongoid::Document>] the cached mongoid documents for the given #value
      def cached_for(field, value)
        cache_key = get_cache_key(field)
        Rails.cache.read(cache_key)[value] || []
      end


      private


        def get_cache_key(field)
          "#{CACHE_KEY_PREFIX}_#{field}"
        end
    end
  end
end

usage.rb

# Caching all items so we can prevent an items query for each group
caching_config = {
  expires_in: 30.seconds, # the whole export shouldn't take that long
  race_condition_ttl: 10.seconds # @see ActiveSupport::Cache::Store
}
Item.cache_all_grouped_by(:group_id, caching_config)

Item.cached_for_group(group.id) is a good suggestion.

cached_for_group it is.

Made the model concern more general.