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

Rails has 2 ways of avoiding the n+1 problem. One involves creating a big join based query to pull in your associations, the other involves making a separate query per association.

When you do includes rails decides which strategy to use for you. It defaults to the separate query approach (preloading) unless it thinks you are using the columns from the associations in you conditions or order. Since that only works with the joins approach it uses that instead.

Rails' heuristics sometimes get it wrong or you may have a specific reason for preferring one approach over the other. preload ( and its companion method eager_load) allow you to specify which strategy you want rails to use.

For detailed understanding refer this blog.

In short:

Preload loads the association data in a separate query.

User.preload(:posts).to_a

# => SELECT "users".* FROM "users" SELECT "posts".* FROM "posts"  WHERE "posts"."user_id" IN (1)

Includes loads the association data in a separate query just like preload. However it is smarter than preload. But in some cases it combines the queries.

How includes is smarter?

we can’t use post table in where condition. Following query will result in an error.

User.preload(:posts).where("posts.desc='ruby is awesome'")

# =>
SQLite3::SQLException: no such column: posts.desc:
SELECT "users".* FROM "users"  WHERE (posts.desc='ruby is awesome')

But we can use post table in where query with include

User.includes(:posts).where('posts.desc = "ruby is awesome"').to_a

# =>
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.desc = "ruby is awesome")

As you can see includes switches from using two separate queries to creating a single LEFT OUTER JOIN to get the data. And it also applied the supplied condition.