N + 2 queries

basic_domain.rb

class Post < ApplicationRecord
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :post
end

domain_belongs_to.rb

class Post < ApplicationRecord
  has_many :comments
  has_many :uncensored_comments, -> { where(censored: false) }, class_name: 'Comment'
  # rails will automatically limit the number of records for us
  has_one :most_liked_comment, -> { order(likes: :desc) }, class_name: 'Comment'
end

domain_with_association.rb

class Post < ApplicationRecord
  has_many :comments
  # we need to specify a new name, a lambda to filter the comments and the model class name
  has_many :uncensored_comments, -> { where(censored: false) }, class_name: 'Comment'
end

n_1_belongs_to.sql

Post Load (0.4ms)  SELECT "posts".* FROM "posts"
Comment Load (0.9ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY "comments"."likes" DESC LIMIT $2  [["post_id", 1], ["LIMIT", 1]]
Comment Load (0.7ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY "comments"."likes" DESC LIMIT $2  [["post_id", 2], ["LIMIT", 1]]
Comment Load (0.6ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY "comments"."likes" DESC LIMIT $2  [["post_id", 3], ["LIMIT", 1]]
Comment Load (0.3ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY "comments"."likes" DESC LIMIT $2  [["post_id", 4], ["LIMIT", 1]]
Comment Load (0.4ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 ORDER BY "comments"."likes" DESC LIMIT $2  [["post_id", 5], ["LIMIT", 1]]

n_1_belongs_to_solved.sql

Post Load (0.4ms)  SELECT "posts".* FROM "posts"
Comment Load (0.4ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2, $3, $4, $5) ORDER BY "comments"."likes" DESC  [["post_id", 1], ["post_id", 2], ["post_id", 3], ["post_id", 4], ["post_id", 5]]

n_1_custom.sql

Post Load (0.5ms)  SELECT  "posts".* FROM "posts"
Comment Load (1.4ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2, $3, $4, $5)  [["post_id", 1], ["post_id", 2], ["post_id", 3], ["post_id", 4], ["post_id", 5]]
Comment Load (6.4ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 1], ["censored", false]]
Comment Load (0.5ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 2], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 3], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 4], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 5], ["censored", false]]

n_1_custom_includes.sql

Post Load (0.5ms)  SELECT  "posts".* FROM "posts"
Comment Load (6.4ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 1], ["censored", false]]
Comment Load (0.5ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 2], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 3], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 4], ["censored", false]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1 AND "comments"."censored" = $2  [["post_id", 5], ["censored", false]]

n_1_custom_solved.sql

Post Load (0.5ms)  SELECT  "posts".* FROM "posts"
Comment Load (0.9ms)  SELECT "comments".* FROM "comments" WHERE "comments"."censored" = $1 AND "comments"."post_id" IN ($2, $3, $4, $5, $6)  [["censored", false], ["post_id", 1], ["post_id", 2], ["post_id", 3], ["post_id", 4], ["post_id", 5]]

n_1_fixed.sql

Post Load (0.5ms)  SELECT  "posts".* FROM "posts"
Comment Load (1.4ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN ($1, $2, $3, $4, $5)  [["post_id", 1], ["post_id", 2], ["post_id", 3], ["post_id", 4], ["post_id", 5]]

n_1_simple.sql

Post Load (0.5ms)  SELECT  "posts".* FROM "posts"
Comment Load (0.8ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 1]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 2]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 3]]
Comment Load (0.2ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 4]]
Comment Load (0.6ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 5]]