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 models?

class Car < ActiveRecord::Base
  belongs_to :store
end

class Store < ActiveRecord::Base
  belongs_to :owner
  has_one :car
end

Answer 1

stores = Store.joins(:car)

This will return all stores for which there is a car. stores[0].car will result in another query.

stores = Store.includes(:car)

This will return all stores, car or no car. stores[0].car will not result in another query.

stores = Store.includes(:car).joins(:car)

This will return all stores with a car. stores[0].car will not result in another query. I wouldn't recommend this for has_many relationships, but it works great for has_one.