Rails Eagerloading

若存在如下Post model：

class Post < ActiveRecord::Base
	belongs_to :author
	has_many :comments
end

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

class Address < ActiveRecord::Base
	belongs_to :addressable, :polymorphic=>true
end
Address.includes(:addressable)
rails
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