FEATURE: option to update default notification level of existing users. (#14084)

Previously, a group's `default_notification_level` change will only affect the users added after it.

Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>

spec/requests/groups_controller_spec.rb

@@ -883,6 +883,67 @@ describe GroupsController do
 
       context "user default notifications" do
         it "should update default notification preference for existing users" do
+          group.update!(default_notification_level: NotificationLevels.all[:watching])
+          user1 = Fabricate(:user)
+          user2 = Fabricate(:user)
+          group.add(user1)
+          group.add(user2)
+          group_user1 = user1.group_users.first
+          group_user2 = user2.group_users.first
+
+          put "/groups/#{group.id}.json", params: {
+            group: {
+              default_notification_level: NotificationLevels.all[:tracking]
+            }
+          }
+
+          expect(response.status).to eq(200)
+
+          expect(group_user1.reload.notification_level).to eq(NotificationLevels.all[:watching])
+          expect(group_user2.reload.notification_level).to eq(NotificationLevels.all[:watching])
+
+          group_users = group.group_users
+          expect(response.parsed_body["user_count"]).to eq(group_users.count)
+
+          group_user1.update!(notification_level: NotificationLevels.all[:regular])
+
+          put "/groups/#{group.id}.json", params: {
+            group: {
+              default_notification_level: NotificationLevels.all[:tracking]
+            }
+          }
+
+          expect(response.status).to eq(200)
+          expect(response.parsed_body["user_count"]).to eq(group.group_users.count - 1)
+          expect(group_user1.reload.notification_level).to eq(NotificationLevels.all[:regular])
+          expect(group_user2.reload.notification_level).to eq(NotificationLevels.all[:watching])
+
+          put "/groups/#{group.id}.json", params: {
+            group: {
+              default_notification_level: NotificationLevels.all[:tracking]
+            },
+            update_existing_users: true
+          }
+
+          expect(response.status).to eq(200)
+          expect(response.parsed_body["success"]).to eq("OK")
+          expect(group_user1.reload.notification_level).to eq(NotificationLevels.all[:regular])
+          expect(group_user2.reload.notification_level).to eq(NotificationLevels.all[:tracking])
+
+          put "/groups/#{group.id}.json", params: {
+            group: {
+              default_notification_level: NotificationLevels.all[:regular]
+            },
+            update_existing_users: false
+          }
+
+          expect(response.status).to eq(200)
+          expect(response.parsed_body["success"]).to eq("OK")
+          expect(group_user1.reload.notification_level).to eq(NotificationLevels.all[:regular])
+          expect(group_user2.reload.notification_level).to eq(NotificationLevels.all[:tracking])
+        end
+
+        it "should update category & tag notification preferences for existing users" do
           user1 = Fabricate(:user)
           user2 = Fabricate(:user)
           CategoryUser.create!(user: user1, category: category, notification_level: 4)