Eager load has many polymorphic association with limit per each owner

has_many_limit_per_likes.rb

# Standart behavior: if you eager load an association with a specified :limit option, it will be ignored, returning all the associated objects
# This example shows how to eager load limited number of records in collection.
# Usage:
#    Post.find(42).likes.size                              # => 300
#    Post.find(42).likes.limit_per_target(5).size          # => 5
#    Post.preload(:likes).map {|t| t.likes.size }          # => [100, 200, 300, ... ]
#    Post.preload(:limited_likes).map {|t| t.likes.size }  # => [5, 5, 5, ... ]
# 
# recommended: postgres ~>9.4, ruby ~>2.2, rails ~>4.2, squeel

# The polymoriphic association model (like) which can be eager-loaded with a specified limit per each owner:
class Like < ActiveRecord::Base
  class WithLimitPerTarget < self
    default_scope ->{ limit_per_target(5) }  # set the limit here
  end
  
  belongs_to :target, polymorphic: true
  
  DIR_OP_MAP = { asc: '>', desc: '<' }
  
  # => SELECT "likes".*, count("lx"."id"), "likes".* FROM "likes" 
  #    LEFT OUTER JOIN (SELECT "likes".* FROM "likes") lx 
  #    ON ("likes"."target_id" = "lx"."target_id" AND "likes"."target_type" = "lx"."target_type" AND "likes"."id" < "lx"."id")
  #    GROUP BY "likes"."id" 
  #    HAVING count("lx"."id") < 5
  def self.limit_per_target(lim = 5, dir = :desc)
    _gt_or_lt_ = DIR_OP_MAP.fetch(dir)
    select( "likes.*, count(lx.id)" ).
    joins  { Like.all.as('lx').    # joins self to count per owner
      on   { (target_id == lx.target_id) & (target_type == lx.target_type) & (id.op _gt_or_lt_, lx.id) }.
      outer}.
    group  { id }.
    having { count(lx.id) < lim }  # use limit
  end
  
  # Concern, which should be included into target model
  concern :HasLikes do
    included do
      has_many :likes, as: :target
      has_many :limited_likes, as: :target, class_name: "Like::WithLimitPerTarget"
      alias_method_chain :likes, :limit
    end
    
    def likes_with_limit
      loaded_limited_likes? ? limited_likes : likes_without_limit
    end
    
    def loaded_limited_likes?
      association(:limited_likes).loaded?
    end
  end
end

# Target model
class Post < ActiveRecord::Base
  include Like::HasLikes
end