FIX: associates email replies using both 'In-Reply-To' and 'References' headers

lib/email/receiver.rb

@@ -211,10 +211,9 @@ module Email
       message_ids.uniq!
       return if message_ids.empty?
 
-      IncomingEmail.where.not(post_id: nil)
-                   .where(message_id: message_ids)
-                   .first
-                   .try(:post)
+      Post.where(id: IncomingEmail.where(message_id: message_ids).select(:post_id))
+          .order(created_at: :desc)
+          .first
     end
 
     def extract_references

spec/components/email/receiver_spec.rb

@@ -236,6 +236,26 @@ describe Email::Receiver do
       expect(emails).to include("someone@else.com", "discourse@bar.com", "team@bar.com", "wat@bar.com")
     end
 
+    it "associates email replies using both 'In-Reply-To' and 'References' headers" do
+      expect { process(:email_reply_1) }.to change(Topic, :count)
+
+      topic = Topic.last
+
+      expect { process(:email_reply_2) }.to change { topic.posts.count }
+      expect { process(:email_reply_3) }.to change { topic.posts.count }
+
+      # Why 6 when we only processed 3 emails?
+      #   - 3 of them are indeed "regular" posts generated from the emails
+      #   - The 3 others are "small action" posts automatically added because
+      #     we invited 3 users (team@bar.com, two@foo.com and three@foo.com)
+      expect(topic.posts.count).to eq(6)
+
+      # trash all but the 1st post
+      topic.ordered_posts[1..-1].each(&:trash!)
+
+      expect { process(:email_reply_4) }.to change { topic.posts.count }
+    end
+
   end
 
   context "new topic in a category" do

spec/fixtures/emails/email_reply_1.eml

@@ -0,0 +1,12 @@
+Return-Path: <one@foo.com>
+From: One <one@foo.com>
+To: team@bar.com
+Cc: two@foo.com, three@foo.com
+Subject: Testing email threading
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <34@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+This is email reply **1**.

spec/fixtures/emails/email_reply_2.eml

@@ -0,0 +1,13 @@
+Return-Path: <two@foo.com>
+From: Two <two@foo.com>
+To: one@foo.com
+Cc: team@bar.com, three@foo.com
+Subject: RE: Testing email threading
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <35@foo.bar.mail>
+In-Reply-To: <34@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+This is email reply **2**.

spec/fixtures/emails/email_reply_3.eml

@@ -0,0 +1,14 @@
+Return-Path: <three@foo.com>
+From: Three <three@foo.com>
+To: one@foo.com
+Cc: team@bar.com, two@foo.com
+Subject: RE: Testing email threading
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <36@foo.bar.mail>
+In-Reply-To: <35@foo.bar.mail>
+References: <34@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+This is email reply **3**.

spec/fixtures/emails/email_reply_4.eml

@@ -0,0 +1,15 @@
+Return-Path: <one@foo.com>
+From: One <one@foo.com>
+To: two@foo.com
+Cc: team@bar.com, three@foo.com
+Subject: RE: Testing email threading
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <37@foo.bar.mail>
+In-Reply-To: <36@foo.bar.mail>
+References: <34@foo.bar.mail>
+  <35@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+This is email reply **4**.