ActiveRecord::Store with JSON column uses nil as default value instead of {}

[sambostock]

Steps to reproduce

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

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

  gem "rails" # , github: "rails/rails", branch: "main"

  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 :nullable_settings, force: true do |t|
    t.json :settings
  end

  create_table :non_nullable_settings, force: true do |t|
    t.json :settings, null: false
  end
end

class BugTest < ActiveSupport::TestCase
  self.test_order = :sorted # Not required, but makes it easier to look at the debug output.

  setup { ActiveRecord::Base.logger.debug(name) }

  test "1_creating nullable settings without settings saves nil" do
    assert_stores_nil(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "nullable_settings"
    end)
  end

  test "2_creating non-nullable settings without settings raises error" do
    assert_raises_not_null_violation(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "non_nullable_settings"
    end)
  end

  test "3_creating nullable settings with attribute default saves empty hash" do
    assert_stores_empty_hash(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "nullable_settings"
      model.attribute :settings, :json, default: {}
    end)
  end

  test "4_creating non-nullable settings with attribute default saves empty hash" do
    assert_stores_empty_hash(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "non_nullable_settings"
      model.attribute :settings, :json, default: {}
    end)
  end

  test "5_creating nullable settings with store saves nil" do
    assert_stores_nil(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "nullable_settings"
      model.store :settings
    end)
  end

  test "6_creating non-nullable settings with store raises error" do
    assert_stores_empty_hash(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "non_nullable_settings"
      model.store :settings
    end) # fails (NotNullViolation)
  end

  test "7_creating nullable settings with store and attribute default saves nil" do
    assert_stores_empty_hash(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "nullable_settings"
      model.attribute :settings, :json, default: {}
      model.store :settings
    end) # fails (nil, not {})
  end

  test "8_creating non-nullable settings with store and attribute default raises error" do
    assert_stores_empty_hash(Class.new(ActiveRecord::Base) do |model|
      model.table_name = "non_nullable_settings"
      model.attribute :settings, :json, default: {}
      model.store :settings
    end) # fails (NotNullViolation)
  end

  private

  def assert_stores_nil(model)                = assert_nil(model.create!.settings_before_type_cast)
  def assert_raises_not_null_violation(model) = assert_raises(ActiveRecord::NotNullViolation) { model.create! }

  def assert_stores_empty_hash(model)
    assert_equal("{}", model.create!.settings_before_type_cast)
  rescue ActiveRecord::NotNullViolation => error
    flunk error.message
  end
end

Expected behavior

When using ActiveRecord::Store with a column of type json, the default value written to the database should be an empty Hash (serialized as an empty JSON object {}), especially if the attribute is marked as having a default value of {}.

Actual behavior

When using ActiveRecord::Store with a column of type json, the default value written to the database is nil.

This is problematic when using non-nullable JSON columns, which cannot accept defaults at the database schema level, as there is no way to avoid violating the NULL constraint.

System configuration

Rails version: rails 7.1.3

Ruby version: ruby 3.3.0