PERF: Allow memory to be freed instead of fetching all the objects into memory at once.

```
MemoryProfiler.report do
  Jobs::UserEmail.new.execute(type: :mailing_list, user_id: user.id)
end.pretty_print
```

Before:
```
Total allocated: 180096119 bytes (1962025 objects)
Total retained:  2194 bytes (16 objects)

allocated memory by gem
-----------------------------------
  66979096  activerecord-4.2.8
  43507184  nokogiri-1.7.1
  43365188  mail-2.6.4
   5960201  activesupport-4.2.8
   5056267  discourse/lib
   4835284  rack-mini-profiler-0.10.1
   3825817  arel-6.0.4
   2186088  i18n-0.8.1
   1719330  discourse/app
```

After:
```
Total allocated: 161935975 bytes (1473940 objects)
Total retained:  2234 bytes (17 objects)

allocated memory by gem
-----------------------------------
  45430264  activerecord-4.2.8
  43568627  nokogiri-1.7.1
  43430754  mail-2.6.4
  11233878  rack-mini-profiler-0.10.1
   5260825  activesupport-4.2.8
   5054491  discourse/lib
   2186088  i18n-0.8.1
   1822494  arel-6.0.4
```

app/mailers/user_notifications.rb

@@ -80,10 +80,21 @@ class UserNotifications < ActionMailer::Base
     @since = opts[:since] || 1.day.ago
     @since_formatted = short_date(@since)
 
-    @new_topic_posts      = Post.mailing_list_new_topics(user, @since).group_by(&:topic) || {}
-    @existing_topic_posts = Post.mailing_list_updates(user, @since).group_by(&:topic) || {}
-    @posts_by_topic       = @new_topic_posts.merge @existing_topic_posts
-    return unless @posts_by_topic.present?
+    topics = Topic
+      .joins(:posts)
+      .includes(:posts)
+      .for_digest(user, 100.years.ago)
+      .where("posts.created_at > ?", @since)
+
+    unless user.staff?
+      topics = topics.where("posts.post_type <> ?", Post.types[:whisper])
+    end
+
+    @new_topics = topics.where("topics.created_at > ?", @since).uniq
+    @existing_topics = topics.where("topics.created_at <= ?", @since).uniq
+    @topics = topics.uniq
+
+    return if @topics.empty?
 
     build_summary_for(user)
     opts = {
@@ -93,6 +104,7 @@ class UserNotifications < ActionMailer::Base
       add_unsubscribe_link: true,
       unsubscribe_url: "#{Discourse.base_url}/email/unsubscribe/#{@unsubscribe_key}",
     }
+
     apply_notification_styles(build_email(@user.email, opts))
   end
 

app/models/post.rb

@@ -77,8 +77,6 @@ class Post < ActiveRecord::Base
 
     q.order('posts.created_at ASC')
   }
-  scope :mailing_list_new_topics, ->(user, since) { for_mailing_list(user, since).where('topics.created_at > ?', since) }
-  scope :mailing_list_updates,    ->(user, since) { for_mailing_list(user, since).where('topics.created_at <= ?', since) }
 
   delegate :username, to: :user
 

app/views/user_notifications/mailing_list.html.erb

@@ -20,21 +20,21 @@
     <td>
       <h4 style="padding: 0 25px; margin: 10px 0;"><%= t('user_notifications.mailing_list.new_topics') %></h3>
       <ul>
-        <% @new_topic_posts.each do |topic, posts| %>
-          <%= mailing_list_topic(topic, posts.length) %>
+        <% @new_topics.each do |topic| %>
+          <%= mailing_list_topic(topic, topic.posts.length) %>
         <% end %>
       </ul>
       <h4 style="padding: 0 25px; margin: 10px 0;"><%= t('user_notifications.mailing_list.topic_updates') %></h3>
       <ul>
-        <% @existing_topic_posts.each do |topic, posts| %>
-          <%= mailing_list_topic(topic, posts.length) %>
+        <% @existing_topics.each do |topic| %>
+          <%= mailing_list_topic(topic, topic.posts.length) %>
         <% end %>
       </ul>
     </td>
   </tr>
 </table>
 <table style='width: 100%; background: #eee;' cellspacing='0' cellpadding='0'>
-  <% @posts_by_topic.each do |topic, posts| %>
+  <% @topics.each do |topic| %>
     <tr>
       <td>
         <h3 style='padding: 20px 20px 10px; margin: 0;'>
@@ -45,7 +45,7 @@
     <%- unless SiteSetting.private_email? %>
       <tr>
         <td style='border-left: 20px solid #eee; border-right: 20px solid #eee; border-bottom: 10px solid #eee; padding: 10px 10px; background: #fff;'>
-          <% posts.each do |post| %>
+          <% topic.posts.each do |post| %>
             <div>
               <img style="float: left; width: 20px; padding-right: 5px;" src="<%= post.user.small_avatar_url %>" title="<%= post.user.username%>">
               <p style="font-size: 15px; color: #888">

app/views/user_notifications/mailing_list.text.erb

@@ -1,28 +1,28 @@
 <%- site_link = raw(@markdown_linker.create(@site_name, '/')) %>
 <%= raw(t 'user_notifications.mailing_list.why', site_link: site_link, date: @since_formatted) %>
 
-<%- if @new_topic_posts.keys.present? -%>
+<%- if @new_topics.present? -%>
   ### <%= t 'user_notifications.mailing_list.new_topics' %>
-  <%- @new_topic_posts.keys.each do |topic| %>
+  <%- @new_topics.each do |topic| %>
     <%= mailing_list_topic_text(topic) %>
   <%- end -%>
 <%- end -%>
 
-<%- if @existing_topic_posts.keys.present? -%>
+<%- if @existing_topics.present? -%>
   ### <%= t 'user_notifications.mailing_list.new_topics' %>
-  <%- @existing_topic_posts.keys.each do |topic| -%>
+  <%- @existing_topics.each do |topic| -%>
     <%= mailing_list_topic_text(topic) %>
-    <%= @existing_topic_posts[topic].length %>
+    <%= topic.posts.length %>
   <%- end -%>
 <%- end -%>
 
 --------------------------------------------------------------------------------
 
 <%- unless SiteSetting.private_email? %>
-  <%- @posts_by_topic.keys.each do |topic| %>
+  <%- @topics.each do |topic| %>
     ### <%= raw(@markdown_linker.create(topic.title, topic.relative_url)) %>
 
-    <%- @posts_by_topic[topic].each do |post| -%>
+    <%- topic.posts.each do |post| -%>
       <%= post.user.name || post.user.username %> - <%= post.created_at %>
   --------------------------------------------------------------------------------
       <%= post.raw %>