Fix: Optional field regex validation in MsgspecDTO (#4195)

[rsamanth1]

Description

Problem

When a field is marked as Optional[Annotated[str, msgspec.Meta(pattern=...)]], the regex constraint validation was bypassed, allowing invalid input to pass through.

Before:

POST / {"url": "htp:/gogle.com"}  # Returns 201 (should be 400)

After:

POST / {"url": "htp:/gogle.com"}  # Returns 400 

Solution

Added post-decoding validation that:

1. Extracts Meta constraints from Optional[Annotated[T, Meta(...)]] types
2. Validates non-None values against these constraints
3. Raises ValidationException for invalid patterns

Changes

• Modified msgspec_dto.py:

  • Added _extract_meta_from_annotation() to unwrap Optional/Annotated types
  • Override decode_builtins() and decode_bytes() to add validation
  • Added _validate_meta_constraints() and _validate_instance() methods

• Added comprehensive tests in test_dto_optional_validation.py:

  • Invalid URLs rejected with 400
  • Valid URLs accepted with 201
  • None values allowed for optional fields
  • Required fields still validate correctly
  • Parametrized tests for various URL patterns

Testing

pytest tests/unit/dto/test_dto_optional_validation.py -v

Closes

[provinzkraut]

I believe this is following msgspec's logic. The field annotation should be Annotated[Optional[str], msgspec.Meta(pattern=...)] not Optional[Annotated[str, msgspec.Meta(pattern=...)]]. Have you verified that it works as intended with the correct annotation?