What's the difference between “includes” and “joins” in ActiveRecord query?

Question

What\'s the difference between \"includes\" and \"joins\" in ActiveRecord query? Can anyone explain to me with the following two associated

Answer 1

:joins is the ActiveRecord version of JOINS, the SQL request.

Store.joins(:car).to_sql
=> SELECT stores.* FROM stores INNER JOIN cars ON cars.store_id = categories.id

So behind the seen it is an implicit INNER JOIN. It will thus indeed return all stores that have a car (because of the inner_join, a left_outer_join would have had a different behaviour). See more here.

Whereas :includes is not a query command. It solves the n+1 query issue by eager_loading the Car model on the Store model. See more one eager_loading here.

stores = Store.includes(:car)

will thus return all stores, and allows to do stores.first.car without firing a new query.