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:
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joinswill 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!
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preloadwill simply preload associations in bulk, using extra queries:User.preload(:posts).to_aQuery:
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.
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eager_loadwill preload associations in bulk, explicitly using aLEFT OUTER JOIN:User.eager_load(:posts).to_aQuery:
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.
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includeswill preload associations likepreload...User.includes(:posts).to_aQuery:
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_aQuery:
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
includesusually 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.
