Disallow calling `#deliver_later` after local message modifications.

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

1 files changed, 18 insertions, 0 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 1109912b04..1fef9eae29 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,21 @@
+*   Disallow calling `#deliver_later` after making local modifications to
+    the message which would be lost when the delivery job is enqueued.
+
+    Prevents a common, hard-to-find bug like
+
+        message = Notifier.welcome(user, foo)
+        message.message_id = my_generated_message_id
+        message.deliver_later
+
+    The message_id is silently lost! *Only the mailer arguments are passed
+    to the delivery job.*
+
+    This raises an exception now. Make modifications to the message within
+    the mailer method instead, or use a custom Active Job to manage delivery
+    instead of using #deliver_later.
+
+    *Jeremy Daer*
+
 *   Removes `-t` from default Sendmail arguments to match the underlying
     `Mail::Sendmail` setting.