through_association_autosave.rb

through_association_autosave.rb

# frozen_string_literal: true

# This script illustrates caveats of ActiveRecord association autosave
# when using a combination of database constrains with a through association (join model)
#
#  ## USAGE
#
#   ruby through_association_autosave.rb
#
#  ## EDGE CASE
#
#  Removing a (seemingly unrelated) validation can result in an exception to be raised when creating a model instance:
#
#  1. When a (has_one) through association item is assigned ActiveRecord will automatically build a join model object
#     (see https://guides.rubyonrails.org/association_basics.html)
#
#  2. When *save* is invoked on the parent model, it will automatically save the associated join object
#     following the association autosave logic:
#     - when autosave is not explicitly defined it will only save new records using `save(validate: true)`
#       and not explicitly raise exceptions if invalid
#       (see https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/autosave_association.rb#L491-L517)
#
#  3. When an unfulfilled database constraint exists (e.g. a NOT NULL constraint) but is not covered by an ActiveRecord validation
#     and the automatically built object is:
#     - valid (in terms of ActiveRecord validations): the save attempt will *raise an exception*
#     - invalid (in terms of ActiveRecord validations): the save attempt will *fail silently* and not raise an exception

###################################
# SETUP
###################################

require "bundler/inline"

##
# GEMS
##
gemfile(true) do
  source "https://rubygems.org"
  gem "activerecord", "6.1"
  gem "sqlite3"
  gem "rspec"
end

##
# CONFIG
##
require "active_record"
require "rspec"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

##
# TABLES
##
ActiveRecord::Schema.define do
  create_table :libraries

  create_table :shelves do |t|
    t.belongs_to :library
    t.boolean :wooden, null: false
  end

  create_table :books do |t|
    t.belongs_to :shelve
  end
end

##
# MODELS
##
class Library < ActiveRecord::Base
  has_many :shelves
end

class Shelve < ActiveRecord::Base
  belongs_to :library
  has_many :books

  attr_accessor :make_invalid
  validate :some_validation

  def some_validation
    errors.add(:base, 'is invalid') if make_invalid
  end
end

class Book < ActiveRecord::Base
  belongs_to :shelve
  has_one :library, through: :shelve
end

###################################
# TEST
###################################

require "rspec/autorun"

RSpec.describe 'autosave validation and constraints bahavior' do
  let(:library) { Library.create! }

  subject(:create_book_with_library) do
    # creating a book in a library will transparently build a shelve via a through association
    Book.new(library: library).tap do |book|
      # save! will not raise an error when the association join model instance is invalid
      book.save!
    end
  end

  context 'when the through item(shelve) is valid' do
    context 'when an unfulfilled database constraint exists' do
      it 'will raise ActiveRecord::NotNullViolation error' do
        expect { create_book_with_library }.to raise_exception(ActiveRecord::NotNullViolation)
      end
    end

    context 'when the database constraint is fulfilled' do
      it 'will persist the join model' do
        book = create_book_with_library

        expect(book.shelve.persisted?).to eq(true)
      end
    end
  end

  context 'when the through item(shelve) is invalid' do
    before do
      allow_any_instance_of(Shelve).to receive(:make_invalid).and_return(true)
    end

    context 'when an unfulfilled database constraint exists' do
      it 'will NOT raise ActiveRecord::NotNullViolation error' do
        expect { create_book_with_library }.not_to raise_exception
      end

      it 'will not persist the join model' do
        book = create_book_with_library

        expect(book.shelve.persisted?).to eq(false)
      end
    end
  end
end