Upgrading to Rails 7.0 changes precision when changing column type to datetime

[willnet]

In Rails 6.1 and below, precision is nil when using change_column to set the column type to datetime without explicit option. In Rails 7.0, precision is 6 even if the migration version is specified as ActiveRecord::Migration[6.1]. This will result in different schemas when running rails db:migrate in Rails 6.1 and Rails 7.0.

#42297 changed the default datetime precision to 6. In Rails7.0, ActiveRecord::Migration[6.1] or lower version includes the following support to maintain the original behavior.

• rails/activerecord/lib/active_record/migration/compatibility.rb

  Line 204 in 2fa5493

  options[:precision] ||= nil

• rails/activerecord/lib/active_record/migration/compatibility.rb

  Line 174 in 2fa5493

  options[:precision] ||= nil

but there seems to be no support for change_column

Steps to reproduce

Using PostgreSQL because I can't reproduce with sqlite3

# 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", "~> 7.0.0"
  gem "pg"
end

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

# docker run --rm -e POSTGRES_PASSWORD=postgres -p 9999:5432 postgres
ActiveRecord::Base.establish_connection("postgres://postgres:postgres@localhost:9999/postgres")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema[6.1].define do
  create_table :posts, force: true do |t|
    t.date :published_at
  end
end

class ChangePublishedAt < ActiveRecord::Migration[6.1]
  def change
    change_column :posts, :published_at, :datetime
  end
end

class Post < ActiveRecord::Base
end

class BugTest < Minitest::Test
  def test_precision_when_change_column_to_datetime
    ChangePublishedAt.migrate(:up)
    Post.reset_column_information
    published_at_column = Post.columns.find { |c| c.name == "published_at" }
    assert_nil published_at_column.precision # fail. Expected 6 to be nil.
  end
end

Expected behavior

the test pass

Actual behavior

the test fails.

System configuration

Rails version:
7.0.7
Ruby version:
3.2.2

[willnet]

For reference, here is the code to check the behavior in Rails 6.1.

# 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", "~> 6.1.0"
  gem "pg"
end

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

# docker run --rm -e POSTGRES_PASSWORD=postgres -p 9999:5432 postgres
ActiveRecord::Base.establish_connection("postgres://postgres:postgres@localhost:9999/postgres")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.date :published_at
  end
end

class ChangePublishedAt < ActiveRecord::Migration[6.1]
  def change
    change_column :posts, :published_at, :datetime
  end
end

class Post < ActiveRecord::Base
end

class BugTest < Minitest::Test
  def test_precision_when_change_column_to_datetime
    ChangePublishedAt.migrate(:up)
    Post.reset_column_information
    published_at_column = Post.columns.find { |c| c.name == "published_at" }
    assert_nil published_at_column.precision # pass
  end
end

[skipkayhil]

Thanks for the great reproduction! I'll have a PR to fix this up shortly