在为模型导轨创建对象之前获取下一个对象ID

Question

I have a Model I need to get the next record id which is going to be create before create that object in my model like:

MyModel.last.id #=> 10
MyModel.last.destroy
MyModel.last.id #=> 9, so (Model.last.id + 1) would be 10... but...
MyModel.create  #=> 11, my next id was actually 11

Can you please suggest a better way to solve my problem?

Thanks

Answer 1

You are looking for the table's AUTO_INCREMENT value. MySQL stores such metadata in the INFORMATION_SCHEMA database. The AUTO_INCREMENT values can be found in the TABLES table. The table contains one entry for each database and table.

I don't think Rails or the MySQL gem provide any built-in method for fetching it.

I have used something like this in one of my previous projects:

# config/initializers/auto_increment.rb

module AutoIncrement
  def auto_increment_value
    connection.execute(<<-SQL.squish).first[0]
      SELECT `AUTO_INCREMENT`
        FROM `INFORMATION_SCHEMA`.`TABLES`
       WHERE `TABLE_SCHEMA` = '#{connection.current_database}'
         AND `TABLE_NAME` = '#{table_name}'
    SQL
  end
end

ActiveRecord::Base.extend(AutoIncrement)

You can then execute:

MyModel.auto_increment_value

and it will return the current value for the table's AUTO_INCREMENT value, eg 11 .

Note that it is not safe to use that value as an explicit ID for your record. You should let MySQL handle the assignment of new IDs – that's what AUTO_INCREMENT is for.

Answer 2

Why worry about the id? If you have some other reasons for a contiguous number, use another field and do some select max on a before_create.

FWIW, MySQL, and Mariadb auto increment only resets on a database restart, which avoids race situations.