搬运工：http://blog.arkency.com/2013/12/rails4-preloading/

Summary

There are 3 ways to do eager loading in Rails:

• #includes
• #preload
• #eager_load

#includes delegates the job to #preload or #eager_load depending on the presence or absence of condition related to one of the preloaded table.

#preload is using separate DB queries to get the data.

#eager_load is using one big query with LEFT JOIN for each eager loaded table.

In Rails 4 you should use #references combined with #includes if you have the additional condition for one of the eager loaded table.