preloading.rb

module Preloading
  def preload(records, association_name, preload_scope)
    return records if records.blank?

    reflection = records.first.association(association_name).reflection

    if reflection.through_reflection
      grouped_scope = reflection.through_reflection.klass.where(reflection.foreign_key => preload_scope).
                                                          includes(reflection.source_reflection_name).
                                                          each_with_object({}) do |r, hash|
        if reflection.collection?
          (hash[r[reflection.through_reflection.foreign_key]] ||= []) << r.association(reflection.source_reflection_name).target
        else
          hash[r[reflection.through_reflection.foreign_key]] = r.association(reflection.source_reflection_name).target
        end
      end

      our_table_key = reflection.association_primary_key
    elsif reflection.belongs_to?
      our_table_key = reflection.foreign_key

      grouped_scope = preload_scope.where(id: records.map { |r| r[our_table_key] }).index_by(&:id)
    else
      our_table_key = reflection.association_primary_key

      if reflection.collection?
        grouped_scope = preload_scope.where(reflection.foreign_key => records).group_by { |r| r[reflection.foreign_key] }
      else
        grouped_scope = preload_scope.where(reflection.foreign_key => records).index_by { |r| r[reflection.foreign_key] }
      end
    end

    records.each do |record|
      binding.pry if association_name == :images
      association = record.association(association_name)

      association.target = grouped_scope[record[our_table_key]]
      association.target ||= [] if reflection.collection?

      association.loaded!
    end

    records
  end
end