FIX: chat was enqueueing too many "chat summary" emails (#31133)

due to an issue with LEFT JOIN, we were enqueue a "chat summary" email
for every new messages in a channel, instead of for every new mentions 😬

This bloated the sidekiq queue with a lot of unecessary jobs as seen in

- https://meta.discourse.org/t/-/347197
- https://meta.discourse.org/t/-/346542

Thankfully, it wasn't sending those emails as the query for listing the
unread mentions and dms was correct when generating the chat summary
email.

plugins/chat/lib/chat/mailer.rb

@@ -44,29 +44,39 @@ module Chat
           AND uo.chat_email_frequency = #{UserOption.chat_email_frequencies[:when_away]}
           AND uo.email_level <> #{UserOption.email_level_types[:never]}
         ), channel_messages AS (
-            SELECT DISTINCT ON (chat_channel_id) chat_channel_id, cm.id AS first_unread_id, user_id AS sender_id
+          SELECT DISTINCT ON (chat_channel_id) chat_channel_id, cm.id AS first_unread_id, user_id AS sender_id
-            FROM chat_messages cm
+          FROM chat_messages cm
-            JOIN users sender ON sender.id = cm.user_id
+          JOIN users sender ON sender.id = cm.user_id
-            WHERE cm.created_at > now() - interval '7 days'
+          WHERE cm.created_at > now() - interval '1 week'
-            AND cm.deleted_at IS NULL 
+          AND cm.deleted_at IS NULL 
-            AND NOT cm.created_by_sdk
+          AND NOT cm.created_by_sdk
-            ORDER BY chat_channel_id, cm.id
+          ORDER BY chat_channel_id, cm.id
-        )
+        ), unread_dms AS (
-        SELECT DISTINCT uccm.user_id
+          SELECT DISTINCT uccm.user_id
-        FROM user_chat_channel_memberships uccm 
+          FROM user_chat_channel_memberships uccm 
-        JOIN chat_channels cc ON cc.id = uccm.chat_channel_id AND cc.deleted_at IS NULL
+          JOIN chat_channels cc ON cc.id = uccm.chat_channel_id AND cc.deleted_at IS NULL AND cc.chatable_type = 'DirectMessage'
-        JOIN channel_messages cm ON cm.chat_channel_id = cc.id AND cm.sender_id <> uccm.user_id
+          JOIN channel_messages cm ON cm.chat_channel_id = cc.id AND cm.sender_id <> uccm.user_id
-        JOIN eligible_users eu ON eu.id = uccm.user_id
+          JOIN eligible_users eu ON eu.id = uccm.user_id AND eu.allow_private_messages
-        LEFT JOIN chat_mentions mn ON mn.chat_message_id = cm.first_unread_id
+          WHERE NOT uccm.muted 
-        LEFT JOIN chat_mention_notifications cmn ON cmn.chat_mention_id = mn.id
+          AND (uccm.last_read_message_id IS NULL OR cm.first_unread_id > uccm.last_read_message_id)
-        LEFT JOIN notifications n ON n.id = cmn.notification_id AND n.user_id = uccm.user_id
+          AND (uccm.last_unread_mention_when_emailed_id IS NULL OR cm.first_unread_id > uccm.last_unread_mention_when_emailed_id)
-        WHERE NOT uccm.muted 
+        ), unread_mentions AS (
-        AND (uccm.last_read_message_id IS NULL OR cm.first_unread_id > uccm.last_read_message_id)
+          SELECT DISTINCT uccm.user_id
-        AND (uccm.last_unread_mention_when_emailed_id IS NULL OR cm.first_unread_id > uccm.last_unread_mention_when_emailed_id)
+          FROM user_chat_channel_memberships uccm 
-        AND (
+          JOIN chat_channels cc ON cc.id = uccm.chat_channel_id AND cc.deleted_at IS NULL AND cc.chatable_type = 'Category'
-            (cc.chatable_type = 'DirectMessage' AND eu.allow_private_messages) OR 
+          JOIN channel_messages cm ON cm.chat_channel_id = cc.id AND cm.sender_id <> uccm.user_id
-            (cc.chatable_type = 'Category' AND uccm.following AND (n.id IS NULL OR NOT n.read))
+          JOIN eligible_users eu ON eu.id = uccm.user_id
+          JOIN chat_mentions mn ON mn.chat_message_id = cm.first_unread_id
+          JOIN chat_mention_notifications cmn ON cmn.chat_mention_id = mn.id
+          JOIN notifications n ON n.id = cmn.notification_id AND n.user_id = uccm.user_id AND NOT n.read
+          WHERE NOT uccm.muted 
+          AND uccm.following
+          AND (uccm.last_read_message_id IS NULL OR cm.first_unread_id > uccm.last_read_message_id)
+          AND (uccm.last_unread_mention_when_emailed_id IS NULL OR cm.first_unread_id > uccm.last_unread_mention_when_emailed_id)
         )
+        SELECT user_id FROM unread_dms
+        UNION
+        SELECT user_id FROM unread_mentions
       SQL
     end
   end

plugins/chat/spec/components/chat/mailer_spec.rb

@@ -53,6 +53,14 @@ describe Chat::Mailer do
   describe "in a followed channel" do
     before { followed_channel.add(user) }
 
+    describe "there is a new message" do
+      let!(:chat_message) { create_message(followed_channel, "hello y'all :wave:") }
+
+      it "does not queue a chat summary" do
+        expect_not_enqueued
+      end
+    end
+
     describe "user is @direct mentioned" do
       let!(:chat_message) do
         create_message(followed_channel, "hello @#{user.username}", Chat::UserMention)
@@ -193,6 +201,14 @@ describe Chat::Mailer do
   describe "in a non-followed channel" do
     before { non_followed_channel.add(user).update!(following: false) }
 
+    describe "there is a new message" do
+      let!(:chat_message) { create_message(non_followed_channel, "hello y'all :wave:") }
+
+      it "does not queue a chat summary" do
+        expect_not_enqueued
+      end
+    end
+
     describe "user is @direct mentioned" do
       before { create_message(non_followed_channel, "hello @#{user.username}", Chat::UserMention) }
 
@@ -221,6 +237,14 @@ describe Chat::Mailer do
   describe "in a muted channel" do
     before { muted_channel.add(user).update!(muted: true) }
 
+    describe "there is a new message" do
+      let!(:chat_message) { create_message(muted_channel, "hello y'all :wave:") }
+
+      it "does not queue a chat summary" do
+        expect_not_enqueued
+      end
+    end
+
     describe "user is @direct mentioned" do
       before { create_message(muted_channel, "hello @#{user.username}", Chat::UserMention) }
 
@@ -247,6 +271,14 @@ describe Chat::Mailer do
   end
 
   describe "in an unseen channel" do
+    describe "there is a new message" do
+      let!(:chat_message) { create_message(unseen_channel, "hello y'all :wave:") }
+
+      it "does not queue a chat summary" do
+        expect_not_enqueued
+      end
+    end
+
     describe "user is @direct mentioned" do
       before { create_message(unseen_channel, "hello @#{user.username}") }
 
@@ -262,6 +294,14 @@ describe Chat::Mailer do
         expect_not_enqueued
       end
     end
+
+    describe "there is an @all mention" do
+      before { create_message(unseen_channel, "hello @all", Chat::AllMention) }
+
+      it "does not queue a chat summary email" do
+        expect_not_enqueued
+      end
+    end
   end
 
   describe "in a direct message" do
@@ -271,7 +311,7 @@ describe Chat::Mailer do
       expect_enqueued
     end
 
-    it "queues a chat summary email when user isn't following the direct message anymore" do
+    it "queues a chat summary email even when user isn't following the direct message anymore" do
       direct_message.membership_for(user).update!(following: false)
       expect_enqueued
     end