Remember why preloading associations "randomly" uses joined tables or multiple queries?

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If you don't like the cleverness of this behavior, you can explicitely tell ActiveRecord how to preload associations Show archive.org snapshot with either JOINs or separate queries:

• 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!

• 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.

• 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.

• 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.