Remember why preloading associations "randomly" uses joined tables or multiple queries?
If you don't like the cleverness of this behavior, you can
explicitely tell ActiveRecord how to preload associations
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with either JOINs or separate queries.
This card gives an overview of the different options to preload associations, but
Which one should I use?
Be careful about which methods you choose for preloading! Please have a look on on our card why you should be super careful with complex eager_load or includes queries.
Thus, as a general guideline.includes or .eager_load with 1-n associations are dangerous. We recommend to mainly use .preload instead.
ActiveRecord::Relation#joins
joins will join the given association so you can order or add conditions.
User.joins(:posts)
Query:
SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
Note that records will not be preloaded that way. Accessing each user's posts will produce extra queries!
ActiveRecord::Relation#preload
preload will simply preload associations in bulk, using extra queries:
User.preload(:posts).to_a
Query:
SELECT "users".* FROM "users"
SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (1)
Accessing each user's posts will not produce extra queries.
If all you want is to preload associations, this is the fastest way, even though it produces additional queries.
ActiveRecord::Relation#eager_load
eager_load will preload associations in bulk, explicitly using a LEFT OUTER JOIN:
User.eager_load(:posts).to_a
Query:
SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0,
"posts"."title" AS t1_r1, "posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3
FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
Note that this will not work when preloading polymorphic associations because of joining.
ActiveRecord::Relation#includes
includes will preload associations like preload ...
User.includes(:posts).to_a
Query:
SELECT "users".* FROM "users"
SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (1)
... and switch to a JOIN if necessary:
User.includes(:posts).where('posts.id > 10').to_a
Query:
SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, "posts"."id" AS t1_r0,
"posts"."title" AS t1_r1,
"posts"."user_id" AS t1_r2, "posts"."desc" AS t1_r3
FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
WHERE (posts.id > 10)
Using includes usually works nicely, but Rails will apply some magic, as mentioned at the beginning of this card.
This is subject to issues with polymorphic associations, too.
