active admin, polymorphic drop down

active_admin.rb

# this is a helper method to provide setters for polymorphic relationships in active admin forms
# attribute_name: name that record knows polymorphic relationship as
# actions: the active record actions that should use this setter, defaults to setter being implemented before create and update
def polymorphic_attribute_set(attribute_name, actions: [:before_create, :before_update])
  klass = config.resource_class
  foreign_key = klass.reflect_on_all_associations.find { |ref| ref.name == attribute_name }.association_foreign_key
  setter = lambda do |rec|
    record = ApplicationRecord.identifier_to_record(params[klass.name.snakecase][foreign_key])
    resource.send("#{attribute_name}=", record)
  end
 
  actions.each { |action| send(action, &setter) }
end

# this is a helper to provide a polymorphic dropdown that will interact with the above polymorphic attribute setter
# f: is the form in which the select is to be placed
# key: is the name of the polymorphic attribute on the record
# options: are the records to be selected from
# selected: is the record to prefill selection with
# display: is the method to call on each option to display in the dropdown
# other options for the select can also be appended as named params
def polymorphic_select(f, key, options, selected: nil, display: :to_s, **opts)
  options_list = options.flatten.map { |opt| [opt.send(display), ApplicationRecord.record_to_identifier(opt)] }
  f.input key, as: :select, collection: options_for_select(options_list, selected&.identifier), **opts
end

admin_foo.rb

ActiveAdmin.register Foo do
  permit_params :bar
  
  # will set bar on record before create and update
  polymorphic_attribute_set :bar
  
  form do |f|
    # adds drop down with list of all Baz and Buzz instances for selection as resource's bar
    polymorphic_select(f, :bar, [Baz.all, Buzz.all], selected: resource.bar)
  end
end

application_record.rb

IDENTIFICATION_JOINER = '>><<'.freeze
def record_to_identifier(record)
  [record.class.name, record.id].join(IDENTIFICATION_JOINER)
end

# WARNING: may raise NameError if identifier cannot be constantized
def self.identifier_to_record(identifier)
  return if identifier.blank?

  record_klass_string, record_id = identifier.split(IDENTIFICATION_JOINER)
  record_klass = record_klass_string.constantize
  record_klass.find_by(record_klass.primary_key => record_id)
end