Add stage_changed timeline events

This adds a new type of timeline event which captures whenever the
stage of an application has changed.

app/models/timeline_event.rb

@@ -75,6 +75,7 @@ class TimelineEvent < ApplicationRecord
          requestable_received: "requestable_received",
          requestable_requested: "requestable_requested",
          reviewer_assigned: "reviewer_assigned",
+         stage_changed: "stage_changed",
          state_changed: "state_changed",
        }
 
@@ -152,11 +153,17 @@ class TimelineEvent < ApplicationRecord
   validates :old_value,
             :new_value,
             presence: true,
-            if: -> { action_required_by_changed? || information_changed? }
+            if: -> do
+              action_required_by_changed? || information_changed? ||
+                stage_changed?
+            end
   validates :old_value,
             :new_value,
             absence: true,
-            unless: -> { action_required_by_changed? || information_changed? }
+            unless: -> do
+              action_required_by_changed? || information_changed? ||
+                stage_changed?
+            end
   validates :column_name, presence: true, if: :information_changed?
   validates :work_history_id,
             :column_name,

spec/models/timeline_event_spec.rb

@@ -91,6 +91,7 @@
         requestable_received: "requestable_received",
         requestable_requested: "requestable_requested",
         reviewer_assigned: "reviewer_assigned",
+        stage_changed: "stage_changed",
         state_changed: "state_changed",
       ).backed_by_column_of_type(:string)
     end
@@ -410,5 +411,28 @@
       it { is_expected.to validate_presence_of(:old_value) }
       it { is_expected.to validate_presence_of(:new_value) }
     end
+
+    context "with a stage changed event type" do
+      before { timeline_event.event_type = :stage_changed }
+
+      it { is_expected.to validate_absence_of(:assignee) }
+      it { is_expected.to validate_absence_of(:old_state) }
+      it { is_expected.to validate_absence_of(:new_state) }
+      it { is_expected.to validate_absence_of(:assessment_section) }
+      it { is_expected.to validate_absence_of(:note) }
+      it { is_expected.to validate_absence_of(:mailer_class_name) }
+      it { is_expected.to validate_absence_of(:mailer_action_name) }
+      it { is_expected.to validate_absence_of(:message_subject) }
+      it { is_expected.to validate_absence_of(:assessment) }
+      it { is_expected.to validate_absence_of(:age_range_min) }
+      it { is_expected.to validate_absence_of(:age_range_max) }
+      it { is_expected.to validate_absence_of(:subjects) }
+      it { is_expected.to validate_absence_of(:requestable_id) }
+      it { is_expected.to validate_absence_of(:requestable_type) }
+      it { is_expected.to validate_absence_of(:work_history_id) }
+      it { is_expected.to validate_absence_of(:column_name) }
+      it { is_expected.to validate_presence_of(:old_value) }
+      it { is_expected.to validate_presence_of(:new_value) }
+    end
   end
 end