ActiveRecord has two query methods to eager load associations, includes and preload. Although the documentation of preload says

Allows preloading of args, in the same way that includes does.

Indeed the two methods have some differences.

In simple words, prefer includes to eager load associations. Use preload only when

  • you want to customize select columns, or
  • you meet error “Can not eagerly load the polymorphic association”.

Both of the two methods eager load associations to avoid N+1 queries. However, includes is more aggressive. If it detects that the associations tables are already joined, through explicitly joins or implicit references in where, it overwrites select fields from the joined query and construct result set from it, rather than loading associations in a separate query. See the generated SQL statement below and the fields aliases like t0_r0.

irb(main):049:0> User.includes(:articles).joins(:articles)
  SQL (0.7ms)  SELECT "users"."id" AS t0_r0,
    "users"."login" AS t0_r1,
    "users"."name" AS t0_r2,
    "users"."created_at" AS t0_r3,
    "users"."updated_at" AS t0_r4,
    "articles"."id" AS t1_r0,
    "articles"."title" AS t1_r1,
    "articles"."content" AS t1_r2,
    "articles"."user_id" AS t1_r3,
    "articles"."created_at" AS t1_r4,
    "articles"."updated_at" AS t1_r5
  FROM "users"
  INNER JOIN "articles"
  ON "articles"."user_id" = "users"."id"

However, if the table is not joined yet, includes will fallback to preload, which loads association in a separate query. See the SQL in log below that articles are eager loaded in another SELECT query.

irb(main):050:0> User.includes(:articles)
  User Load (0.1ms)  SELECT "users".* FROM "users"
  Article Load (0.2ms)  SELECT "articles".* FROM "articles"
    WHERE "articles"."user_id" IN (1, 2)

Because includes may (or may not) overwrite select, if you have your own select clause, use preload instead. See example below:

users = User.joins(:articles).select(
  'users.*, articles.created_at as last_posted_at'
# NoMethodError: undefined method `last_posted_at'
# => "2013-08-23 11:20:36.536968"

Older version of Rails has trouble to includes polymorphic associations. It seems the newer version of Rails is smart enough to fallback to preload. However if you still see the error “Can not eagerly load the polymorphic association”, try switching to preload.