Prepare to drop `EmailLog#topic_id`.

app/models/email_log.rb

@@ -1,6 +1,8 @@
 require_dependency 'distributed_mutex'
 
 class EmailLog < ActiveRecord::Base
+  self.ignored_columns = %w{topic_id}
+
   CRITICAL_EMAIL_TYPES ||= Set.new %w{
     account_created
     admin_login
@@ -14,7 +16,7 @@ class EmailLog < ActiveRecord::Base
 
   belongs_to :user
   belongs_to :post
-  belongs_to :topic
+  has_one :topic, through: :post
 
   validates :email_type, :to_address, presence: true
 

lib/email/receiver.rb

@@ -634,7 +634,7 @@ module Email
     def forwarded_reply_key?(email_log, user)
       incoming_emails = IncomingEmail
         .joins(:post)
-        .where('posts.topic_id = ?', email_log.topic_id)
+        .where('posts.topic_id = ?', email_log.topic.id)
         .addressed_to(email_log.reply_key)
         .addressed_to_user(user)
         .pluck(:to_addresses, :cc_addresses)

lib/email/sender.rb

@@ -77,11 +77,9 @@ module Email
       # always set a default Message ID from the host
       @message.header['Message-ID'] = "<#{SecureRandom.uuid}@#{host}>"
 
-      if topic_id.present?
-        email_log.topic_id = topic_id
-
-        post = Post.find_by(id: post_id)
-        topic = Topic.find_by(id: topic_id)
+      if topic_id.present? && post_id.present?
+        post = Post.find_by(id: post_id, topic_id: topic_id)
+        topic = post.topic
         first_post = topic.ordered_posts.first
 
         topic_message_id = first_post.incoming_email&.message_id.present? ?

spec/components/email/sender_spec.rb

@@ -316,7 +316,7 @@ describe Email::Sender do
       it 'should create the right log' do
         email_sender.send
         expect(email_log.post_id).to eq(post.id)
-        expect(email_log.topic_id).to eq(topic.id)
+        expect(email_log.topic.id).to eq(topic.id)
       end
     end