ActiveRecord migrations with polymorphic references create indexes with different name

[ClearlyClaire]

Steps to reproduce

Create a table with a polymorphic references in a < 6.0 migration, run the migration with ActiveRecord 6.

This was most probably introduced by e8437a6.

Minimal testcase

# 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", "6.1.3"
  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)

class CreateSettings < ActiveRecord::Migration[5.0]
  def self.up
    create_table :settings do |t|
      t.references :target, null: false, polymorphic: true
    end
  end
end

class BugTest < Minitest::Test
  def test_migration_up
    CreateSettings.migrate(:up)
    assert CreateSettings.index_name_exists?(:settings, :index_settings_on_target_type_and_target_id)
  end
end

Expected behavior

The created index should have the same name when the migration runs in ActiveRecord 6 than it had in ActiveRecord 5, in this case, index_settings_on_target_type_and_target_id.

Actual behavior

The migration creates an index with a different name: index_settings_on_target.

System configuration

Rails version: 6.1.3

Ruby version: 2.7.2p137

[andreyruby]

Same issue #41547

[zzak]

@ClearlyClaire @aglushkov Could you give #42350 a try and let us know if that solves your issue?

[rails-bot]

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 6-1-stable branch or on main, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.