Reproduction script for Issue #1372

REPRODUCTION_SCRIPT.rb

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
  gem "factory_bot", "~> 5.0"
  gem "activerecord"
  gem "sqlite3"
end

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

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.string :title
  end

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

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Post < ApplicationRecord
  has_many :comments, dependent: :destroy
end

class Comment < ApplicationRecord
  belongs_to :post
end

FactoryBot.define do
  factory :post do
    after(:build) do |post|
      p post.comments
    end

    title { "xxx" }
  end

  factory :comment do

  end
end

class FactoryBotTest < Minitest::Test
  def test_comments_destroy_factorybot
    Comment.destroy_all
    post = FactoryBot.create(:post)
    3.times { FactoryBot.create(:comment, post: post) }

    assert_equal 3, Comment.count
    post.destroy!
    assert_equal 0, Comment.count
  end

  def test_comments_destroy_factorybot_2
    Comment.destroy_all
    post = Post.new
    p post.comments # This test passes if you comment this line out
    post.save!
    3.times do
      comment = Comment.new
      comment.post = post
      comment.save!
    end

    assert_equal 3, Comment.count
    post.destroy!
    assert_equal 0, Comment.count
  end
end