Active Record counter_cache + inverse_of bug

bug.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.integer :comments_count, default: 0
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments

  # Tests pass if we disable inverse_of:
  # has_many :comments, inverse_of: false
end

class Comment < ActiveRecord::Base
  belongs_to :post, counter_cache: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    other_post = Post.create!
    comment = Comment.create!

    assert_equal 0, post.comments_count
    assert_equal 0, other_post.comments_count

    post.comments << comment

    assert_equal 1, post.reload.comments_count

    comment.update(post_id: other_post.id)

    # These fail if the comments association inverse_of set.
    # That is because `stale_target?` returns `false` when the target was set
    # via `inverse_of`, and so `BelongsToAssociation#update_counters` ends up
    # incrementing and then decrementing the `comments_count` on the original
    # target, rather than incrementing on the new target and decrementing on the
    # old one.
    assert_equal 0, post.reload.comments_count
    assert_equal 1, other_post.reload.comments_count
  end
end

# Rails tests this behavior, but happens to do it using associations where automatic inverse_of is disabled because of a custom foreign_key:
# https://github.com/rails/rails/blob/main/activerecord/test/cases/associations/has_many_associations_test.rb#L1443-L1459