"Unknown key: :polymorphic" error on Rails master (6.1.0.alpha)

[filippoliverani]

When using current Rails master, acts_as_nested_set fails on load with this error:

An error occurred while loading ./spec/models/users_spec.rb.
Failure/Error: belongs_to :parent, options

ArgumentError:
  Unknown key: :polymorphic. Valid keys are: :class_name, :anonymous_class, :primary_key, :foreign_key, :dependent, :validate, :inverse_of, :autosave, :required, :touch, :counter_cache, :optional, :default
# ./lib/awesome_nested_set/awesome_nested_set.rb:105:in `acts_as_nested_set_relate_parent!'
# ./lib/awesome_nested_set/awesome_nested_set.rb:55:in `acts_as_nested_set'
# ./spec/support/models.rb:2:in `<class:Note>'
# ./spec/support/models.rb:1:in `<top (required)>'
# ./spec/spec_helper.rb:22:in `<top (required)>'
# ./spec/models/users_spec.rb:1:in `<top (required)>'

This is due to :polimorphic key option being added only when set to true since
osmaelo/rails@2c008d9

[glaszig]

this is an issue with rails which was fixed yesterday, see rails/rails#40879.
you can either use the 6-1-stable branch or try the following monkey patch:

# config/initializers/active_record.rb

module PolymorphicBelongsTo
  def valid_options(options)
    valid = super + [:polymorphic, :counter_cache, :optional, :default]
    valid += [:foreign_type] if options[:polymorphic]
    valid += [:ensuring_owner_was] if options[:dependent] == :destroy_async
    valid
  end
end

ActiveSupport.on_load :active_record do
  ActiveRecord::Associations::Builder::BelongsTo.extend PolymorphicBelongsTo
end