gistfile1.rb

has_one :display_gallery_item, as: :imageable, class_name: "GalleryItem"

class ServiceCenterPreloader
  class GalleryItemPreloader < ActiveRecord::Associations::Preloader::HasOne
    def initialize(owners, reflection)
      super(GalleryItem, owners, reflection, ActiveRecord::Relation.new(nil, nil))
    end

  private
    def build_scope
      t = klass.table_name
      distinct = "#{t}.imageable_id, #{t}.imageable_type"
      scope = super
      scope.select_values = ["DISTINCT ON(#{distinct}) #{t}.*"]
      scope.order(distinct)
    end
  end

  def self.preload(service_centers)
    records = Array.wrap(service_centers.to_a).compact.uniq

    reflection = ServiceCenter.reflections[:display_gallery_item]
    GalleryItemPreloader.new(records, reflection).run
  end
end

service_centers = ServiceCenter.all.to_a
ServiceCenterPreloader.preload(service_centers)