rails Eager loading

     若存在如下Post model：

使用下面的循环加载数据时产生了N+1查询问题：

Post.all.each do |post|
  puts "Post:            " + post.title
  puts "Written by:      " + post.author.name
  puts "Last comment on: " + post.comments.first.created_on
end

首先，解决author获取问题：

Post.includes(:author).each do |post|

   然后解决comments加载：

Post.includes(:author, :comments).each do |post|

   带条件的eager loading：

Post.includes([:author, :comments]).where(['comments.approved = ?', true]).all

  多态关系的eager laoding

Address.includes(:addressable)