FIX: If a group is deleted also remove assignments (#592)

This fix addresses an issue if a group is deleted topics will remain
assigned to all group members with no way to un-assign those users from
the original group assignment.

Now if a group is deleted any group assignments are also deleted.

plugin.rb

@@ -872,6 +872,16 @@ module ::DiscourseAssign
     end
   end
 
+  on(:group_destroyed) do |group, user_ids|
+    User
+      .where(id: user_ids)
+      .find_each do |user|
+        user.notifications.for_assignment(group.assignments.select(:id)).destroy_all
+      end
+
+    Assignment.active_for_group(group).destroy_all
+  end
+
   register_search_advanced_filter(/in:assigned/) do |posts|
     posts.where(<<~SQL) if @guardian.can_assign?
         topics.id IN (

spec/plugin_spec.rb

@@ -146,5 +146,32 @@
         expect_job_enqueued(job: Jobs::AssignNotification, args: { assignment_id: assignment.id })
       end
     end
+
+    describe "on 'group_destroyed'" do
+      let(:group) { Fabricate(:group) }
+      let(:user) { Fabricate(:user) }
+      let(:first_assignment) { Fabricate(:topic_assignment, assigned_to: group) }
+      let(:second_assignment) { Fabricate(:post_assignment, assigned_to: group) }
+
+      before do
+        group.users << user
+        Fabricate(
+          :notification,
+          notification_type: Notification.types[:assigned],
+          user: user,
+          data: { assignment_id: first_assignment.id }.to_json,
+        )
+        Fabricate(
+          :notification,
+          notification_type: Notification.types[:assigned],
+          user: user,
+          data: { assignment_id: second_assignment.id }.to_json,
+        )
+      end
+
+      it "removes user's notifications related to group assignments" do
+        expect { group.destroy }.to change { user.notifications.assigned.count }.by(-2)
+      end
+    end
   end
 end