Flaky!

comment.rb

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

flaky.rb

require 'bundler/inline'

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

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

  gem 'activerecord', '6.1.1'
  gem 'sqlite3'
  gem 'rspec'
  gem 'pry'
end

require 'active_record'
require 'rspec'

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, null: false
  end

  create_table :comments, force: true do |t|
    t.belongs_to :post
  end
end

autoload :Post, './post.rb'
autoload :Comment, './comment.rb'

RSpec.describe Post do
  it 'updates something, whatever' do
    post = Post.create!
    post.update!(comments_count: 42)

    expect(post.reload.comments_count).to eq(42)
  end

  it 'doesnt really do anything' do
    expect(Comment.count).to eq(0)
  end
end

RSpec::Core::Runner.invoke

post.rb

class Post < ActiveRecord::Base
  has_many :comments
end