When use with create_list, build method returns same instance every time

[shrkw]

Description

I want to create Post record and Tag records on one transaction due to schema constraint and validation.
So, I wrote FactoryBot.create(:post, tags: [FactoryBot.build(:tag)]) and works perfect.
However when create multiple records by call create_list, FactoryBot.build(:tag) returns same instance every time, so Tag records are updated multiple times.

D, [2021-06-17T09:38:20.196778 #46727] DEBUG -- :   TRANSACTION (0.1ms)  begin transaction
D, [2021-06-17T09:38:20.197197 #46727] DEBUG -- :   Post Create (0.1ms)  INSERT INTO "posts" DEFAULT VALUES
D, [2021-06-17T09:38:20.198598 #46727] DEBUG -- :   Tag Create (0.2ms)  INSERT INTO "tags" ("post_id") VALUES (?)  [["post_id", 1]]
D, [2021-06-17T09:38:20.199523 #46727] DEBUG -- :   TRANSACTION (0.2ms)  commit transaction
D, [2021-06-17T09:38:20.202593 #46727] DEBUG -- :   TRANSACTION (0.2ms)  begin transaction
D, [2021-06-17T09:38:20.203601 #46727] DEBUG -- :   Post Create (0.5ms)  INSERT INTO "posts" DEFAULT VALUES
D, [2021-06-17T09:38:20.205742 #46727] DEBUG -- :   Tag Update (0.2ms)  UPDATE "tags" SET "post_id" = ? WHERE "tags"."id" = ?  [["post_id", 2], ["id", 1]]
D, [2021-06-17T09:38:20.206156 #46727] DEBUG -- :   TRANSACTION (0.2ms)  commit transaction

Is this designed behavior?

Reproduction Steps

A reproduction script is here.

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
  gem "factory_bot", "6.2.0"
  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|
  end
  create_table :tags, force: true do |t|
    t.references :post, foreign_key: true, null: false
  end
end

class Post < ActiveRecord::Base
  has_many :tags, dependent: :destroy
  validates :tags, presence: true
end

class Tag < ActiveRecord::Base
  belongs_to :post
end

FactoryBot.define do
  factory :post do
  end
  factory :tag do
  end
end

class FactoryBotTest < Minitest::Test
  def test_create_post_with_children_on_single_transaction
    post = FactoryBot.create(:post, tags: [FactoryBot.build(:tag)])
    assert_equal post.tags.count, 1
  end
  def test_create_post_list_with_children_on_single_transaction
    posts = FactoryBot.create_list(:post, 2, tags: [FactoryBot.build(:tag)])

    refute_equal posts[0].tags, posts[1].tags
  end
end

Expected behavior

build returns different instance every time if it is called with *_list method.

Actual behavior

build returns same instance.

System configuration

factory_bot version: 6.2.0
rails version: 6.0.3.7
ruby version: 2.7.2

[composerinteralia]

I see why this is confusing, but I don't think it's a bug in factory_bot. An equivalent way of writing this in ruby that I think makes the problem a little clearer:

tag = FactoryBot.build(:tag)
FactoryBot.create_list(:post, 2, tags: [tag])

We build a tag, and then we pass that one tag to create_list. With this example, I would expect for each post to have the same tag. There's nothing here that would suggest create_list should create new tags along the way.

If you want to execute different code for each element in the list, I believe you can do that in a block passed to create_list. I'm not sure how well documented that is, but it might look something like:

FactoryBot.create_list(:post, 2) do
  post.update!(tags: [FactoryBot.build(:tag)])
end

[shrkw]

Hmm... your example cannot pass validates :tags, presence: true validation.

  1) Error:
FactoryBotTest#test_create_post_list_with_children_on_single_transaction:
ActiveRecord::RecordInvalid: Validation failed: Tags can't be blank
    /usr/local/bundle/gems/activerecord-6.1.3.2/lib/active_record/validations.rb:80:in `raise_validation_error'
    /usr/local/bundle/gems/activerecord-6.1.3.2/lib/active_record/validations.rb:53:in `save!'
    /usr/local/bundle/gems/activerecord-6.1.3.2/lib/active_record/transactions.rb:302:in `block in save!'

Should I use prevent create_list method?

    posts = [
      FactoryBot.create(:post, tags: [FactoryBot.build(:tag)]),
      FactoryBot.create(:post, tags: [FactoryBot.build(:tag)])
    ]

[composerinteralia]

Ah right, I missed your validation (I'm used to tags being a many-to-many relationship rather than a many-to-one). In that case you might consider one of the various solutions we have for has_many associations so that you can do FactoryBot.create(:post) and have factory_bot create the required tags. Avoiding create_list is certainly an option as well, but you would still need to build the tags every time you create a post.