Class: RuboCop::Cop::Isucon::Mysql2::NPlusOneQuery

Inherits:

Base

• Object
• Base
• RuboCop::Cop::Isucon::Mysql2::NPlusOneQuery

Extended by:

AutoCorrector

Includes:

RuboCop::Cop::Isucon::Mixin::Mysql2XqueryMethods, RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods

Defined in:

lib/rubocop/cop/isucon/mysql2/n_plus_one_query.rb

Overview

Note:

If Database isn't configured, auto-correct will not be available. (Only offense detection can be used)

Note:

For the number of N+1 queries that can be detected by this cop, there are too few that can be corrected automatically

Checks that there’s no N+1 query

Examples:

# bad
reservations = db.xquery('SELECT * FROM `reservations` WHERE `schedule_id` = ?', schedule_id).map do |reservation|
  reservation[:user] = db.xquery('SELECT * FROM `users` WHERE `id` = ? LIMIT 1', id).first
  reservation
end

# good
rows = db.xquery(<<~SQL, schedule_id)
  SELECT
    r.id AS reservation_id,
    r.schedule_id AS reservation_schedule_id,
    r.user_id AS reservation_user_id,
    r.created_at AS reservation_created_at,
    u.id AS user_id,
    u.email AS user_email,
    u.nickname AS user_nickname,
    u.staff AS user_staff,
    u.created_at AS user_created_at
  FROM `reservations` AS r
  INNER JOIN users u ON u.id = r.user_id
  WHERE r.schedule_id = ?
SQL

# bad
courses.map do |course|
  teacher = db.xquery('SELECT * FROM `users` WHERE `id` = ?', course[:teacher_id]).first
end

# good
# This is similar to ActiveRecord's preload
# c.f. https://guides.rubyonrails.org/active_record_querying.html#preload
courses.map do |course|
  @users_by_id ||= db.xquery('SELECT * FROM `users` WHERE `id` IN (?)', courses.map { |course| course[:teacher_id] }).each_with_object({}) { |v, hash| hash[v[:id]] = v }
  teacher = @users_by_id[course[:teacher_id]]
end

Constant Summary

Constants included from RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods

RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods::ENUMERABLE_METHOD_NAMES, RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods::LOOP_TYPES, RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods::MSG, RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods::POST_CONDITION_LOOP_TYPES

Constants included from RuboCop::Cop::Isucon::Mixin::Mysql2XqueryMethods

RuboCop::Cop::Isucon::Mixin::Mysql2XqueryMethods::NON_STRING_WARNING_MSG

Method Summary

Methods included from RuboCop::Cop::Isucon::Mixin::NPlusOneQueryMethods

#on_send

Methods included from RuboCop::Cop::Isucon::Mixin::DatabaseMethods

#connection, #enabled_database?, #find_table_name_from_column_name

Methods included from RuboCop::Cop::Isucon::Mixin::Mysql2XqueryMethods

#find_xquery, #with_db_query

Methods included from RuboCop::Cop::Isucon::Mixin::OffenceLocationMethods

#offense_location