gistfile1.rb

require 'rom/memory'

class BlenderRelation < ROM::Relation[:memory]
  repository :memory
  register_as :blender

  forward :join
end

class PostWithTagsMapper < ROM::Mapper
  relation :blender
  register_as :with_tags

  model PostWithTag
end

class TagRelation < ROM::Relation[:sql]
  dataset :tags
  one_to_many :taggings, key: :tag_id

  def for_posts
      association_join(:taggings)                                                                                                                                                                                     
      .where(taggings__taggable_type: 'Post',                                                                                                                                                      
            taggings__taggable_id:   Array(announcements).map{|a| a[:id] })                                                                                                                                         
      .select(:name, :taggable_id)   
  end
end

class TagListMapper < ROM::Mapper
  attribute :id, from: :taggable_id
  group :tags do
    attribute :name
  end
end
  
class TagBlender
  attr_reader :rom
  
  def initialize(rom = ROM.env)
    @rom = rom
  end

  def with_tags(posts, tag_list)
    rom.relation(:blender).join(posts.to_a, (posts >> tag_list).to_a)
  end

end

blender       = TagBlender.new(ROM.env)

posts         = ROM.env.relation(:posts).page(2)
tags          = ROM.env.relation(:tags).as(:tag_list)

blender.with_tags(posts, tags).as(:with_tag)