Post.joins(:comments)
=>
Post Load (21.3ms)  SELECT  "posts".* FROM "posts" INNER JOIN "comments" ON "comments"."post_id" = "posts"."id"

Post.preload(:comments).map { |post| post.comments }
=>
Post Load (0.7ms)  SELECT "posts".* FROM "posts"
Comment Load (17.7ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13)

Post.preload(:comments).where("comments.id=1")
=>
ActiveRecord::StatementInvalid (PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "comments")

Post.eager_load(:comments)
=>
SELECT  DISTINCT "posts"."id" FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" LIMIT $1  [["LIMIT", 11]]
SELECT "posts"."id" AS t0_r0, "posts"."title" AS t0_r1, "posts"."description" AS t0_r2, "posts"."user_id" AS t0_r3, "posts"."created_at" AS t0_r4, "posts"."updated_at" AS t0_r5, "posts"."status" AS t0_r6, "comments"."id" AS t1_r0, "comments"."content" AS t1_r1, "comments"."post_id" AS t1_r2, "comments"."created_at" AS t1_r3, "comments"."updated_at" AS t1_r4 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" WHERE "posts"."id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11)  [["id", 1], ["id", 2], ["id", 3], ["id", 4], ["id", 5], ["id", 6], ["id", 7], ["id", 8], ["id", 9], ["id", 10], ["id", 11]]

Post.includes(:comments)
=>
Post Load (4.1ms)  SELECT  "posts".* FROM "posts" LIMIT $1  [["LIMIT", 11]]
Comment Load (0.5ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11)