FIX: makes every_month use the same weekday

plugin.rb

@@ -600,7 +600,17 @@ after_initialize do
         when 'every_day'
           recurrence = 'FREQ=DAILY'
         when 'every_month'
-          recurrence = 'FREQ=MONTHLY'
+          start_date = event.starts_at.beginning_of_month.to_date
+          end_date = event.starts_at.end_of_month.to_date
+          weekday = event.starts_at.strftime('%A')
+
+          count = 0
+          (start_date..end_date).each do |date|
+            count += 1 if date.strftime('%A') == weekday
+            break if date.day == event.starts_at.day
+          end
+
+          recurrence = "FREQ=MONTHLY;BYDAY=#{count}#{weekday.upcase[0, 2]}"
         when 'every_weekday'
           recurrence = 'FREQ=DAILY;BYDAY=MO,TU,WE,TH,FR'
         else
@@ -609,7 +619,6 @@ after_initialize do
         end
 
         next_starts_at = RRuleGenerator.generate(recurrence, event.starts_at)
-
         difference = event.ends_at - event.starts_at
         next_ends_at = next_starts_at + difference.seconds
 

spec/acceptance/recurrence_spec.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+require_relative '../fabricators/event_fabricator'
+
+describe 'discourse_post_event_recurrence' do
+  let(:user_1) { Fabricate(:user, admin: true) }
+  let(:topic_1) { Fabricate(:topic, user: user_1) }
+  let(:post_1) { Fabricate(:post, topic: topic_1) }
+  let(:starts_at) { Time.zone.parse('2020-09-10 19:00') }
+  let(:post_event_1) { Fabricate(:event, post: post_1, starts_at: starts_at, ends_at: starts_at + 1.hour) }
+
+  before do
+    freeze_time(starts_at)
+
+    SiteSetting.calendar_enabled = true
+    SiteSetting.discourse_post_event_enabled = true
+  end
+
+  context 'every_month' do
+    before do
+      post_event_1.update!(recurrence: 'every_month')
+    end
+
+    it 'sets the next month at the same weekday' do
+      DiscourseEvent.trigger(:discourse_post_event_event_ended, post_event_1)
+
+      expect(post_event_1.starts_at).to eq_time(Time.zone.parse('2020-10-08 19:00'))
+    end
+  end
+
+  context 'every_week' do
+    before do
+      post_event_1.update!(recurrence: 'every_week')
+    end
+
+    it 'sets the next week at the same weekday' do
+      DiscourseEvent.trigger(:discourse_post_event_event_ended, post_event_1)
+
+      expect(post_event_1.starts_at).to eq_time(Time.zone.parse('2020-09-17 19:00'))
+    end
+  end
+
+  context 'every_day' do
+    before do
+      post_event_1.update!(recurrence: 'every_day')
+    end
+
+    it 'sets the next day' do
+      DiscourseEvent.trigger(:discourse_post_event_event_ended, post_event_1)
+
+      expect(post_event_1.starts_at).to eq_time(Time.zone.parse('2020-09-11 19:00'))
+    end
+  end
+
+  context 'every_weekday' do
+    before do
+      post_event_1.update!(
+        starts_at: Time.zone.parse('2020-09-11 19:00'),
+        ends_at: Time.zone.parse('2020-09-11 19:00') + 1.hour,
+        recurrence: 'every_weekday'
+      )
+    end
+
+    it 'sets the next day' do
+      DiscourseEvent.trigger(:discourse_post_event_event_ended, post_event_1)
+
+      expect(post_event_1.starts_at).to eq_time(Time.zone.parse('2020-09-14 19:00'))
+    end
+  end
+end