Insta: Snapshot Testing for Ruby @ Rocky Mountain Ruby 2026, Boulder, Colorado

Transcript

1. Marco Roth Insta: Snapshot Testing for Ruby Sep 29, 2026 - Rocky Mountain Ruby, Boulder, Colorado
2. Extracted from the Herb Test Suite
5. assert_equal where the expected value manages itself.
6. Where you want to assert the whole thing, without having to write an assertion for every detail.
7. UserSerializer.render(user)
8. UserSerializer.render(user) - - : : '{"id":42,"name":"Ada","role":"a dmin","created_at":"2026 01 15T0 9 30 00Z"}'
9. require "test_helper" class UserSerializerTest < Minitest::Test def test_user_json expected = '{"id":42,"name":"Ada","role":"admin","created_at":" assert_equal UserSerializer.render(ADA), expected end def test_admin_json expected = '{"id":7,"name":"Grace","role":"owner","created_at": assert_equal UserSerializer.render(GRACE), expected end end
11. Now, the UserSerializer, changes, and we also want the avatar_url serialized
12. So you update your UserSerializer and run your tests
13. rake test
15. Now, we would have to go through all tests and add avatar_url, even though we can see that the actual value was right
16. Lets abort here and revert this avatar_url change for now
17. This is where Insta comes in
18. require "test_helper" class UserSerializerTest < Minitest::Test def test_user_json assert_snapshot UserSerializer.render(ADA) end def test_admin_json assert_snapshot UserSerializer.render(GRACE) end end
19. rake test
22. Whenever an assertion fails, it writes a new pending .snap.new file
23. insta review
25. Accepting will write the snapshot file to the test/snapshots/ folder
27. So lets add our new avatar_url into the serializer again.
28. rake test
30. Since the assertions failed it wrote some new pending .snap.new files
31. insta review
33. This will update the snapshot file accordingly
35. Inline Snapshots assert_inline_snapshot()
36. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end
37. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end insta review
38. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse) end end
39. require "test_helper" class InlineTest < Minitest::Test def test_reverse assert_inline_snapshot("Hello World".reverse, "dlroW olleH") end end
40. Technically, you could alias assert_inline_snapshot to assert_equal to have this apply to all asserts in your codebase
41. ✓ Minitest + RSpec ✓ file snapshots (.snap + YAML frontmatter, source metadata) ✓ inline snapshots (Prism-powered file rewriting) ✓ interactive review CLI (review · accept · reject · pending · status · clean) ✓ difftastic structural diffs ✓ serializers: to_s · inspect · json · yaml ✓ redactions (jq-like selectors) ✓ INSTA_UPDATE modes + CI auto-detection ✓ per-snapshot metadata (input · description · info)
42. Where snapshots shine: •API responses (the full JSON) •rendered HTML views •ASTs •compiled output •CLI output •error messages
43. Seems to be the perfect test companion for the AI age
44. gem "insta"
46. Thank you for the insta gem name 🙏 Renan Garcia
47. Feedback Welcome!
48. Happy Testing
49. l b g g M t Thank you 🙏 @marcoroth marcoroth.dev @marcoroth_ @marcoroth.dev @marcoroth@ruby.social /in/marco-roth