What's the difference between “left_joins” and “includes” in Rails 5?

问题

In Rails 5, they have added the ActiveRecord query method left_joins. What's the difference between left_joins and includes? I've always seen includes as a left join.

回答1:

includes by default loads the association data in 2 queries just like preload. But with additional references call it switches from using two separate queries to creating a single LEFT OUTER JOIN like left_joins.

Refs:

Preload, Eagerload, Includes and Joins

Making sense of ActiveRecord joins, includes, preload, and eager_load

来源：https://stackoverflow.com/questions/53414830/whats-the-difference-between-left-joins-and-includes-in-rails-5