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wire 2026-06-01
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ACTION MAILER TEST ASSERTIONS FIXED FOR NESTED ATTACHMENTS

By RepoJournal · Filed · About Rails · Composed from the cited sources · methodology

55728
1 person shipped this
55728 @55728 1 cited

The assert_part and assert_no_part helpers can now see body parts buried inside multipart/alternative containers when attachments are present.

Rails shipped a critical fix [1] to ActionMailer's test assertions that were silently missing nested body parts in messages with both text and HTML bodies alongside attachments. The bug lived in assert_part and assert_no_part [2], which only inspected top-level Mail#parts and couldn't see body parts wrapped in multipart/alternative containers nested below the top-level multipart/mixed part. This broke email testing for the most common real-world scenario: a message with both plain text and HTML versions plus an attachment. The fix [3] changes the assertion logic to traverse the full MIME tree instead of stopping at the surface. If you've written ActionMailer tests since 8.2.0.alpha, your assertions may have been passing falsely - upgrade immediately to catch real failures.

Action items

References

  1. [1] Fix assert_part/assert_no_part for body parts nested under an attachment rails/rails ↗
  2. [2] Fix `assert_part` / `assert_no_part` for body parts nested under an attachment ↗ rails/rails
  3. [3] Merge pull request #57521 from 55728/fix-assert-part-nested-multipart rails/rails ↗

$ ls rails/reviews/ # the step-back reads

the week in review Ractor-ready Active Record and Active Job land in Rails the month in review Herb becomes Rails 8.2 default
Full archive → Weekly reviews →

Quick answers

What shipped in Rails on June 1, 2026?
The assert_part and assert_no_part helpers can now see body parts buried inside multipart/alternative containers when attachments are present. In total, 6 commits and 3 pull requests landed.
Who contributed to Rails on June 1, 2026?
1 developer shipped this update, including 55728.
What were the notable Rails updates?
Fix assert_part/assert_no_part for body parts nested under an attachment, Fix `assert_part` / `assert_no_part` for body parts nested under an attachment, and Merge pull request #57521 from 55728/fix-assert-part-nested-multipart.
♥

$ log @rails --recent

Attribute-read regression fixed, decimal numericality corrected 79 shipped in_order_of double-serializes values, id_value alias restored 50 shipped Inlined attribute writers, leaner ActiveRecord::Key 31 shipped MySQL 5.6 falls off the support floor as Arel stops rebuilding literals 27 shipped

$ log --all # elsewhere on the wire

Alignment & Post-training verl standardizes low-precision rollout kernels on DeepGEMM Agent Security Guardrails exposes hooks over HTTP, certifi floor raised in agent-scan

$ ls showcases/

Safety Evals AI Red Teaming Agent Security Interpretability Alignment & Post-training

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