user_lazy.rb

# app/models/post.rb
def user_lazy
  BatchLoader.for(user_id).batch do |user_ids|
    User.where(id: user_ids)
  end
end

posts = Post.where(id: [1, 2, 3]) # SELECT * FROM posts WHERE id IN (1, 2, 3)

users = posts.map { |post| post.user_lazy }

users.each { |user| puts "#{user.name}" } # SELECT * FROM users WHERE id IN (1, 2, 3)