How to build dynamic enums in Rails 6 model concerns?

I'm trying to build a Bookable model concern that adds an enum to the including model, that's used for tracking the stage of the booking:

module Bookable
  extend ActiveSupport::Concern

  STAGES = {
    confirmed: 0,
    completed: 1,
    cancelled: 2,
    issue_raised: 3
  }.freeze

  included do
    enum stage: STAGES.merge(self.extra_stages)

    belongs_to :customer
    belongs_to :provider
    validates :stage, presence: true

    def self.extra_stages
      {}
    end
  end
end

The STAGES constant defines available stages, however I want including models to be able to add stages that are specific to them, by overriding the self.extra_stages method. For example, in a Mission model:

class Mission < ApplicationRecord
  include Bookable

  def self.extra_stages
    {
      awaiting_estimate: 4,
      awaiting_payment: 5,
      awaiting_report: 6,
      report_sent: 7
    }
  end
end

However, this code fails:

$ bundle exec rails c
Loading development environment (Rails 6.0.2.2)
[1] pry(main)> Mission.stages
NoMethodError: undefined method `extra_stages' for Mission (call 'Mission.connection' to establish a connection):Class
Did you mean?  extract_associated
from /home/gueorgui/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activerecord-6.0.2.2/lib/active_record/dynamic_matchers.rb:22:in `method_missing'

Any clue as to what I might be doing wrong? Thank you in advance!

There are cleaner ways to do what I'm about to present, but it will accomplish what you're trying to do. Mind the brevity (I removed your associations to do a quick spot-check on my machine):

module Bookable
  extend ActiveSupport::Concern
  included do 
    STAGES = {
      confirmed: 0,
      completed: 1,
      cancelled: 2,
      issue_raised: 3
    }.freeze
  end 
end

class ApplicationRecord < ActiveRecord::Base 
  self.abstract_class = true 

  def self.acts_as_bookable_with(extra_stages = {})
    include Bookable

    enum stage: self::STAGES.merge(extra_stages)
  end
end 

class Mission < ApplicationRecord
  acts_as_bookable_with({
      awaiting_estimate: 4,
      awaiting_payment: 5,
      awaiting_report: 6,
      report_sent: 7
    })
end

If you want to define these on the class, it would look like this:

module Bookable
  extend ActiveSupport::Concern
  included do 
    STAGES = {
      confirmed: 0,
      completed: 1,
      cancelled: 2,
      issue_raised: 3
    }.freeze
  end 
end

class ApplicationRecord < ActiveRecord::Base 
  self.abstract_class = true 

  def self.acts_as_bookable_with(extra_stages)
    include Bookable
    if extra_stages.is_a?(Symbol)
      extra_stages = self.send(extra_stages)
    elsif extra_stages.is_a?(Hash)
      # do nothing
    else
      raise TypeError, "can't find extra_stages from #{extra_stages.inspect}"
    end
    stages = self::STAGES.merge(extra_stages)
    enum stage: stages
  end
end 

class Comment < ApplicationRecord
  def self.extra_stages
    {
      awaiting_estimate: 4,
      awaiting_payment: 5,
      awaiting_report: 6,
      report_sent: 7
    }
  end

  acts_as_bookable_with(:extra_stages)

end

Note that we're calling acts_as_bookable_with after we define our class method. Otherwise, we'll get undefined method error.

There isn't a whole lot of "bad" in having this in ApplicationRecord. It's not the most ideal way of doing it, but most of these acts_as_* modules follow this exact pattern anyway and inject into ActiveRecord::Base.