Cache not expiring when model has empty update in transaction

[TheDro]

We've encountered a situation where the cache isn't expired if, within a transaction, a model has a "non-update" followed by an update. Here's the code that created a stale cache in our application:

def update_user(id, attributes) # hash
  ApplicationRecord.transaction do
    User.find(id).update(attributes.except(:email))
    update_sign_in_information(id, attributes[:email])
  end
end

def update_sign_in_information(user_id, email)
  user = User.find(user_id)
  user.update(email: email)
  # other stuff
end

If only the email attribute is passed in, this leads to a stale cache. Here are some more examples that demonstrate the bug:

class Tester
  def self.test
    reset
    ApplicationRecord.transaction do
      user = User.first
      user.update({})
      user.update(name: "test2")
    end
    puts "###### user name: #{User.fetch(User.first.id).name}" # test2, good

    reset
    ApplicationRecord.transaction do
      User.first.update({})
      User.first.update(name: "test2")
    end
    puts "###### user name: #{User.fetch(User.first.id).name}" # test1, stale

    reset
    ApplicationRecord.transaction do
      User.first.update(name: "test1")
      User.first.update(name: "test2")
    end
    puts "###### user name: #{User.fetch(User.first.id).name}" # test1, stale

  end

  def self.reset
    User.first.update(name: "test1")
    User.fetch(User.first.id) # fill the cache
  end
end

A similar "bug" exists in ActiveRecord:

class User < ApplicationRecord
  after_commit :print_name

  def print_name
    puts user.name
  end
end

ApplicationRecord.transaction do
  User.first.update(name: "test1")
  User.first.update(name: "test2")
end

# prints test1 after the whole transaction is committed

The bug goes pretty deep and seems to involve the activerecord and ar_transaction_changes gems. Essentially, the @transaction_changed_attributes variable that IdentityCache's _run_commit_callbacks method is checking is attached to the record, but the after_commit callback is only being called on the first instance even if the transaction contains multiple updates. Here's the monkey patch my team is currently considering adding to address the issue, but could lead to performance regressions:

def _run_commit_callbacks
  # if destroyed? || transaction_changed_attributes.present?
    expire_cache
  # end
  super
end