facet_expander.rb

# Converts facets with foreign keys to their corresponding objects.
# Usage:
# 
# Each article belongs to a user, and has a facet for the user_id attribute.
#
#   facets = Article.facets
#   FacetExpander.expand facets, :user
#
# facets[:users] has facets for each user object, and original user_id facet is
# still available as well.
# 
class FacetExpander
  def self.expand(facets, reference)
    new(facets, reference).expand
  end

  def initialize(facets, reference)
    @facets, @reference = facets, reference
  end

  def expand
    facets[expanded_key] = {}
    facets[id_key].each do |id, count|
      object = collection.detect { |instance| instance.id == id }
      next if object.nil?

      facets[expanded_key][object] = count
    end
  end

  private

  attr_reader :facets, :reference

  def collection
    @collection ||= klass.where id: facets[id_key].keys
  end

  def expanded_key
    reference.to_s.pluralize.to_sym
  end

  def id_key
    "#{reference}_id".to_sym
  end

  def klass
    @klass ||= reference.to_s.camelize.constantize
  end
end