Display description validation error in milestones

We had a validation rule for milestones which made sure either the
status or the description was present. However, since the description is
now translatable, the validation error wasn't being displayed in the
admin form.

Moving the validation rule to the translation object fixes the problem.
However, the translation object needs to check an attribute in the
milestone object in order to know whether the description is required or
not. This is tricky because by default it loads the milestone object
from the database, meaning it doesn't work with new records and it
ignores params sent by the browser.

The solution is to manually assign the globalized model before
validating the object. It's a hack, but apparently Rails doesn't provide
a better way to handle this case [1].

[1] https://github.com/rails/rails/issues/32024

app/models/milestone.rb

@@ -13,7 +13,9 @@ class Milestone < ActiveRecord::Base
 
   validates :milestoneable, presence: true
   validates :publication_date, presence: true
-  validate :description_or_status_present?
+
+  before_validation :assign_milestone_to_translations
+  validates_translation :description, presence: true, unless: -> { status_id.present? }
 
   scope :order_by_publication_date, -> { order(publication_date: :asc, created_at: :asc) }
   scope :published,                 -> { where("publication_date <= ?", Date.current) }
@@ -23,9 +25,9 @@ class Milestone < ActiveRecord::Base
     80
   end
 
-  def description_or_status_present?
-    unless description.present? || status_id.present?
-      errors.add(:description)
+  private
+
+    def assign_milestone_to_translations
+      translations.each { |translation| translation.globalized_model = self }
     end
-  end
 end