background

What happened to this code when I wrote this code? So I read the actual Rails code.

What I want to do is make sure to call the external API only once for one Hoge and save the result in Fuga.

class Hoge < ActiveRecord::Base
  has_oen :fuga

  def piyo
    return fuga if fuga.present? # <-I don't want to lock it, so check if it's related in advance
    with_lock do
      reload # <-I want to clear the cache of fuga * 1
      return fuga if fuga.present?

      result = call_api
      create_fuga(**result)
    end
  end

  private

  def call_api
    # call api
  end
end

class Fuga < ActiveRecord::Base
  belongs_to :hoge
end

Initially, there is no reload of * 1 and fuga is cached, so a timing issue may occur. I thought, I couldn't reproduce the error case with spec, so I read the Rails code.

https://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html

# https://github.com/rails/rails/blob/914caca2d31bd753f47f9168f2a375921d9e91cc/activerecord/lib/active_record/locking/pessimistic.rb#L82-L90

# Wraps the passed block in a transaction, locking the object
# before yielding. You can pass the SQL locking clause
# as argument (see <tt>lock!</tt>).
def with_lock(lock = true)
  transaction do
    lock!(lock)
    yield
  end
end

So see lock!

# https://github.com/rails/rails/blob/914caca2d31bd753f47f9168f2a375921d9e91cc/activerecord/lib/active_record/locking/pessimistic.rb#L63-L80

# Obtain a row lock on this record. Reloads the record to obtain the requested
# lock. Pass an SQL locking clause to append the end of the SELECT statement
# or pass true for "FOR UPDATE" (the default, an exclusive row lock). Returns
# the locked record.
def lock!(lock = true)
  if persisted?
    if has_changes_to_save?
      raise(<<-MSG.squish)
        Locking a record with unpersisted changes is not supported. Use
        `save` to persist the changes, or `reload` to discard them
        explicitly.
      MSG
    end

    reload(lock: lock)
  end
  self
end

I see At the time of persisted ?, reload is read internally. .. No wonder the test doesn't fail. That's why I realized that I didn't need to add reload, so I added only the spec case.

Relationship between ActiveRecord with_lock and cache

background