SQL statement with subquery is consistent but not atomic.

test.rb

#!/usr/bin/env ruby

require 'mysql2'

#------------------------------------
# config
#------------------------------------
MYSQL_CONF = {
  host: 'localhost',
  username: 'root',
  password: '',
  database: 'test'
}

#------------------------------------
# setup
#------------------------------------
Mysql2::Client.new(MYSQL_CONF).tap do |cli|
  cli.query 'drop table if exists likes;'
  cli.query 'drop table if exists photos;'
  cli.query 'drop table if exists users;'
  cli.query 'create table likes ( user_id INT, photo_id INT, created_at timestamp default current_timestamp, primary key (user_id, photo_id) );'
  cli.query 'create table photos ( id INT AUTO_INCREMENT PRIMARY KEY, likes_count INT );'
  cli.query 'create table users ( id INT AUTO_INCREMENT PRIMARY KEY, name varchar(255) );'

  cli.query 'insert into photos set likes_count = 0;'
  cli.query "insert into users set name = 'A';"
end

#------------------------------------
# like action
#------------------------------------
def like(user_id, photo_id)

  cli = Mysql2::Client.new(MYSQL_CONF)

  begin
    p 'begin!!'
    cli.query 'BEGIN'

    cli.query <<-EOS
      update photos
      set likes_count = likes_count+1
      where not exists (
        select 1 from likes where user_id = #{user_id} and photo_id = #{photo_id}
      );
    EOS

    cli.query <<-EOS
      insert into likes (user_id, photo_id)
      select #{user_id}, #{photo_id}
      from dual
      where not exists (
        select 1 from likes
        where user_id = #{user_id}
        and photo_id = #{photo_id}
      );
    EOS

    cli.query 'COMMIT'
    p 'commit!!!'
  rescue => e
    cli.query 'ROLLBACK'
    p 'rollback!!!'
    p e
  end
end

#------------------------------------
# 3 parallel request
#------------------------------------
(1..3).map do
  Thread.new {
    like(1, 1)
  }
end.each do |t|
  t.join
end

#------------------------------------
# result
#------------------------------------
Mysql2::Client.new(MYSQL_CONF).tap do |cli|
  p '== likes =='
  cli.query('select * from likes;').each do |row| p row; end

  p '== photos =='
  cli.query('select * from photos;').each do |row| p row; end
end