実行回数：クエリ一回
associationをLEFT OUTER JOINでまとめて取得している。
→実行後にテーブルの絞り込みができる。
例：
Post.all.eager_load(:user)
# SELECT "posts"."id" AS t0_r0, "posts"."name" AS t0_r1, "posts"."user_id" AS t0_r2, "posts"."created_at" AS t0_r3, "posts"."updated_at" AS t0_r4, "users"."id" AS t1_r0, "users"."name" AS t1_r1, "users"."created_at" AS t1_r2, "users"."updated_at" AS t1_r3 FROM "posts" LEFT OUTER JOIN "users" ON "users"."id" = "posts"."user_id"絞り込みができる例：
Post.eager_load(:user).where(user: { id: 1 })
# SELECT "posts"."id" AS t0_r0, "posts"."name" AS t0_r1, "posts"."user_id" AS t0_r2, "posts"."created_at" AS t0_r3, "posts"."updated_at" AS t0_r4, "user"."id" AS t1_r0, "user"."name" AS t1_r1, "user"."created_at" AS t1_r2, "user"."updated_at" AS t1_r3 FROM "posts" LEFT OUTER JOIN "users" "user" ON "user"."id" = "posts"."user_id" WHERE "user"."id" = ? [["id", 1]]関連ページ
includes
preload
N+1問題
includesメソッドとpreloadメソッドとeager_loadメソッドの使い分け