specs for previous

app/mailers/email_reply.rb

@@ -13,14 +13,14 @@ class EmailReply < ApplicationMailer
     end
 
     # threading
-    headers "Message-Id" => @comment.mailing_list_message_id
-    if @comment.parent_comment.present?
-      headers "In-Reply-To" => @comment.parent_comment.mailing_list_message_id
-    end
-    headers "References" => (
-      ([@comment.story.mailing_list_message_id] + @comment.parents.map(&:mailing_list_message_id))
-      .map { |r| "<#{r}>" }
-    )
+    headers "Message-Id" => @comment.mailing_list_message_id,
+      "References" => (
+        ([@comment.story.mailing_list_message_id] + @comment.parents.map(&:mailing_list_message_id))
+        .map { |r| "<#{r}>" }
+      ),
+      "In-Reply-To" => @comment.parent_comment.present? ?
+        @comment.parent_comment.mailing_list_message_id :
+        @comment.story.mailing_list_message_id
 
     mail(
       to: user.email,

spec/mailers/email_reply_spec.rb

@@ -3,6 +3,29 @@
 require "rails_helper"
 
 RSpec.describe EmailReply, type: :mailer do
+  it "has a stable message-id" do
+    comment = create(:comment)
+    user = comment.user
+
+    e1 = EmailReply.reply(comment, user)
+    e2 = EmailReply.reply(comment, user)
+    expect(e1["Message-ID"]).to_not be_nil
+    expect(e1["Message-ID"].to_s).to eq(e2["Message-ID"].to_s)
+  end
+
+  it "has parent in-reply-to and in references" do
+    user = create(:user)
+    comment = create(:comment)
+    reply = create(:comment, parent_comment: comment)
+
+    email = EmailReply.reply(reply, user)
+    expect(email["Message-ID"].to_s).to eq("<#{reply.mailing_list_message_id}>")
+    expect(email["In-Reply-To"].to_s).to eq("<#{comment.mailing_list_message_id}>")
+    expect(email["References"].to_s).to include(comment.mailing_list_message_id)
+    # test inconsistency - factory will create reply with different story id
+    expect(email["References"].to_s).to include(reply.story.mailing_list_message_id)
+  end
+
   it "addresses replies to receiver" do
     comment = create(:comment)
     user = comment.user