Scoping within first_or_create callback

first_or_create_callback.rb

# frozen_string_literal: true

require "bundler/inline"

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

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

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "5.2.0"
  gem "sqlite3", "~> 1.3.6"
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|
  end

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

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post

  after_save :access_comments_in_callback

  def access_comments_in_callback
    $sql = Post.first.comments.to_sql
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!(content: 'My Comment')

    second_comment = post.comments.where(content: 'My Second Comment').first_or_initialize
    second_comment.save!
    assert_equal 'SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = 1', $sql

    post.comments.where(content: 'My Third Comment').first_or_create!
    assert_equal 'SELECT "comments".* FROM "comments" WHERE "comments"."post_id" = 1', $sql
    # Instead we get:  SELECT "comments".* FROM "comments" WHERE "comments"."content" = 'My Third Comment' AND "comments"."post_id" = 1
  end
end