undelete assignment override on child undelete

When a deleted AssignmentOverrideStudent is undeleted, also undelete the
parent AssignmentOverride if it is deleted.

fixes GRADE-849

Test plan:
- Create a course with at least one student.
- Add an assignment, but assign it to only one student.
- Unenroll that student from the course.
- Reenroll the student.
- Edit the assignment from above; check that it is still listed as
  assigned to the student from before, and that changing any
  parameters of the assignment (such as the due date) completes
  successfully instead of throwing an error.

Change-Id: I7cf56adeb2479dc5f7dec225aebb84ef9d1d0145
Reviewed-on: https://gerrit.instructure.com/141873
Tested-by: Jenkins
Reviewed-by: Shahbaz Javeed <sjaveed@instructure.com>
Reviewed-by: Keith T. Garner <kgarner@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>
QA-Review: Indira Pai <ipai@instructure.com>
Product-Review: Keith T. Garner <kgarner@instructure.com>

app/models/assignment_override.rb

@@ -165,6 +165,12 @@ class AssignmentOverride < ActiveRecord::Base
     end
   end
 
+  def undestroy(active_state: 'active')
+    self.workflow_state = active_state
+    save!
+    true
+  end
+
   scope :active, -> { where(:workflow_state => 'active') }
 
   scope :visible_students_only, -> (visible_ids) do

app/models/assignment_override_student.rb

@@ -27,6 +27,7 @@ class AssignmentOverrideStudent < ActiveRecord::Base
   after_create :update_cached_due_dates
   after_destroy :update_cached_due_dates
   after_destroy :destroy_override_if_needed
+  after_update :undelete_override_if_deleted
   before_validation :default_values
   before_validation :clean_up_assignment_if_override_student_orphaned
 
@@ -82,6 +83,11 @@ class AssignmentOverrideStudent < ActiveRecord::Base
   end
   protected :destroy_override_if_needed
 
+  def undelete_override_if_deleted
+    assignment_override.undestroy if workflow_state_was == 'deleted' && active? && assignment_override&.deleted?
+  end
+  protected :undelete_override_if_deleted
+
   def self.clean_up_for_assignment(assignment)
     return unless assignment.context_type == "Course"
     return if assignment.new_record?

spec/models/assignment_override_student_spec.rb

@@ -146,6 +146,16 @@ describe AssignmentOverrideStudent do
     expect(@ao.reload).to be_deleted
   end
 
+  it 'undeletes its override if it is undeletd and the override is deleted' do
+    adhoc_override_with_student
+
+    @override_student.destroy
+    @ao.reload
+
+    @override_student.undestroy
+    expect(@ao.reload).not_to be_deleted
+  end
+
   describe "clean_up_for_assignment" do
     it "if callbacks aren't run clean_up_for_assignment should delete invalid overrides" do
       adhoc_override_with_student