Tags cannot be removed with `tag_ids` after calling `as_json`

[iberianpig]

Summary

After calling the as_json method on a model, attempting to remove tags using label_ids = [] does not work - the tags remain attached to the model.

Environment

• acts-as-taggable-on: v6.0.0 ~ v12.0.0
• Rails: 5-2-stable ~ 8.0-stable
• Ruby: 2.7.8 ~ 3.4.1

Steps to Reproduce

1. Create a model with custom tag class (Label)
2. Set up acts_as_taggable_on :labels
3. Add tags using label_ids
4. Call as_json method
5. Try to remove tags using label_ids = []

Reproduction Code

• Save following code as issue_cannot_save_tags_after_as_json.rb in repository root.
• $ ruby issue_cannot_save_tags_after_as_json.rb

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end
gemfile = File.read(File.expand_path("Gemfile", __dir__)).strip
gemfile(true) do
  source "https://rubygems.org"

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

  # gem "rails", github: "rails/rails", branch: "5-1-stable"
  # gem "rails", github: "rails/rails", branch: "5-2-stable"
  # gem "rails", github: "rails/rails", branch: "6-0-stable"
  # gem "rails", github: "rails/rails", branch: "6-1-stable"
  # gem "rails", github: "rails/rails", branch: "7-0-stable"
  # gem "rails", github: "rails/rails", branch: "7-1-stable"
  # gem "sqlite3", '~> 1.4.4'
  
  gem "rails", github: "rails/rails", branch: "8-0-stable"
  gem "sqlite3", '~> 2.1.0'
  gem 'debug'
  gem 'pry-byebug'

  gem 'stringio'

  gem 'acts-as-taggable-on', path: '.'
end

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

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

# Run acts-as-taggable-on migrations
ActiveRecord::Schema.define do
  # Create acts-as-taggable-on tables
  create_table :tags, force: true do |t|
    t.string :name
    t.integer :taggings_count, default: 0
    t.string :type
  end
  add_index :tags, ['name'], name: 'index_tags_on_name' # , unique: true

  create_table :taggings, force: true do |t|
    t.integer :tag_id

    t.string :taggable_type
    t.integer :taggable_id

    t.string :tagger_type
    t.integer :tagger_id

    t.string :context, limit: 128

    t.string :tenant , limit: 128

    t.datetime :created_at
  end

  add_index :taggings,
            ['tag_id', 'taggable_id', 'taggable_type', 'context', 'tagger_id', 'tagger_type'],
            unique: true, name: 'taggings_idx'
  add_index :taggings, :tag_id , name: 'index_taggings_on_tag_id'

  # Test model tables
  create_table :articles, force: true do |t|
    t.string :title
    t.text :content
    t.timestamps

    t.text :cached_label_list
  end

end


# Model definitions
class Article < ActiveRecord::Base
  acts_as_taggable_on :labels

  # # Adding this fixes the issue
  # def as_json(options = {})
  #   # Exclude label_list
  #   super(options.merge(except: :label_list))
  # end
end

class Label < ActsAsTaggableOn::Tag
end

# Test class
class ActsAsTaggableOnTest < Minitest::Test

  def test_update_article_with_empty_labels
    article = Article.create!(title: "Test Title", content: "Test Content")

    # Create labels
    label1 = Label.find_or_create_by!(name: "Label1")
    label2 = Label.find_or_create_by!(name: "Label2")

    # Update article (set labels)
    article.update!(
      label_ids: [label1.id, label2.id]
    )

    article = Article.find(article.id)

    article.reload

    # ↓ Removing this makes the test pass
    article.as_json # Accesses article.label_list, causing INSERT/DELETE operations in after_save

    # label_ids immediately performs INSERT/DELETE on labels + then enters after_save
    article.update!(label_ids: [])

    article.reload

    assert_empty article.label_list
    # ActsAsTaggableOnTest#test_update_article_with_empty_labels [issue_cannot_save_tags_after_as_json.rb:126]:
    # Expected ["Label1", "Label2"] to be empty.
  end
end

Expected Behavior

Tags should be completely removed when setting label_ids = []

Actual Behavior

After calling as_json, tags are not removed when using label_ids = []

Workarounds

Exclude label_list from as_json

class Article < ActiveRecord::Base
  acts_as_taggable_on :labels
  
  def as_json(options = {})
    super(options.merge(except: :label_list))
  end
end

Analysis

• The as_json method accesses label_list, which modifies the internal state
• Subsequent updates using label_ids cause INSERT/DELETE operations in the after_save callback to malfunction
• Using label_list works correctly

Additional Information

See the attached issue_cannot_save_tags_after_as_json.rb for the complete reproduction test code.