Gem Load Error is: uninitialized constant ActsAsTree::InstanceMethods::ActiveRecord

[jensb]

I am getting this error when using gem 'acts_as_tree' in my Rails project's Gemfile. (Rails 5.2.4, Ruby 2.5.0, on Ubuntu Linux), also when adding a , git: line to point to this repository.
Why is the gem assuming "ActiveRecord" is a local constant inside the module? Even ::ActiveRecord does not seem to help.
acts_as_tree worked fine when I was using Rails 4.2 and Ruby 1.9.3 before starting the upgrade.

$ bundle exec rails c
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:84:in `rescue in block (2 levels) in require': There was an error while trying to load the gem 'acts_as_tree'. (Bundler::GemRequireError)
Gem Load Error is: uninitialized constant ActsAsTree::InstanceMethods::ActiveRecord
Did you mean?  ActiveModel
Backtrace for gem load error is:
/home/jens/.bundle/ruby/2.5.0/acts_as_tree-2e2890a7c082/lib/acts_as_tree.rb:355:in `<module:InstanceMethods>'
/home/jens/.bundle/ruby/2.5.0/acts_as_tree-2e2890a7c082/lib/acts_as_tree.rb:242:in `<module:ActsAsTree>'
/home/jens/.bundle/ruby/2.5.0/acts_as_tree-2e2890a7c082/lib/acts_as_tree.rb:3:in `<top (required)>'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:81:in `require'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:81:in `block (2 levels) in require'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:76:in `each'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:76:in `block in require'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:65:in `each'
/usr/lib/ruby/vendor_ruby/bundler/runtime.rb:65:in `require'
/usr/lib/ruby/vendor_ruby/bundler.rb:114:in `require'

[felixbuenemann]

Looks like something with your Rails upgrade went bad or you’re requiring the gem before activerecord.

[jensb]

Weird, my application.rb looks like this:

# -*- encoding : utf-8 -*-
require File.expand_path('../boot', __FILE__)

require 'rails/all'

# If you have a Gemfile, require the gems listed there, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(:default, Rails.env=='selenium' ? 'test' : Rails.env) if defined?(Bundler)

module Foobar
  class Application < Rails::Application
     .....
  end
end

This looks OK to me and it worked with Rails 4.2.1. What is wrong here?