Rails single-table inheritance issue reproduction

test.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", "6.0.0"
  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.string :name
  end

  create_table :taxonomies, force: true do |t|
    t.string :type
    t.string :post_id
  end
end

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

class Post < ApplicationRecord
  has_many :taxonomies, dependent: :destroy

  accepts_nested_attributes_for :taxonomies
end

class Taxonomy < ApplicationRecord
  belongs_to :post

  validates :type, presence: true
end

class Category < Taxonomy; end
class Tag < Taxonomy; end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.taxonomies << Category.create!

    assert_equal 1, post.taxonomies.count
    assert_equal 1, Category.count
    assert_equal post.id, Category.first.post.id
  end
end