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.

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The Wire · Showcase

ACTION MAILER TEST ASSERTIONS FIXED FOR NESTED ATTACHMENTS

By RepoJournal · Filed 06:00 UTC on June 1, 2026 · About Rails

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

→ Update to latest rails main to pick up the nested multipart assertion fix rails/rails [plan]
→ Re-run your ActionMailer test suite after upgrading to surface any previously hidden assertion failures rails/rails [plan]

References

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

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.

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