fix(dto): apply msgspec constraints to the constrained union member - #4988
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Transfer models wrap the type of optional and partial fields in a union with `None`/`UnsetType`, but the constraints extracted from the source model were attached to the union itself, producing annotations such as `Annotated[Union[str, UnsetType], Meta(min_length=8)]`. msgspec only accepts type specific constraints on the type they apply to and raises a `TypeError` for those, so any DTO with a constrained optional field or a `partial` config failed to build. The metadata is now applied to the single constrainable union member instead, which also makes dropping length constraints for partial DTOs - a workaround for the same limitation - unnecessary. Closes litestar-org#4181
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## main #4988 +/- ##
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+ Coverage 67.41% 67.42% +0.01%
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Files 293 293
Lines 15308 15316 +8
Branches 1736 1738 +2
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+ Hits 10320 10327 +7
- Misses 4839 4841 +2
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`_annotate_with_meta` only moves the metadata onto a union member when there is exactly one member that can carry it. The fallback path, where the union has more than one such member and the metadata stays on the union itself, had no test. Add one so the branch is exercised.
| if field_definition.passthrough_constraints: | ||
| if (field_meta := _create_struct_field_meta_for_field_definition(field_definition)) is not None: | ||
| field_type = Annotated[field_type, field_meta] | ||
| field_type = _annotate_with_meta(field_type, field_meta) |
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Wouldn't it be easier to check field_definition.is_partial than do the type unwrapping? iiuc this case only happens for partial fields where we add an outer Union. In other cases, we do not alter the type, so a wrongly attached Meta is a user error, which we should not silently fix.
Remove the _annotate_with_meta helper and its extra imports (get_args, is_union, NoneType). Apply the constraint annotation to the inner type before wrapping in Union only when field_definition.is_partial is set, which is the only case where we add the outer Union ourselves. For all other cases the user's own annotation is untouched (a wrongly attached Meta is a user error).
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Transfer models wrap optional and partial fields in a union with
NoneorUnsetType, but constraints copied from the source model were attached to the union itself, giving annotations likeAnnotated[Union[str, UnsetType], Meta(min_length=8)]. msgspec only accepts type specific constraints on the type they apply to and raises a TypeError for that, so any DTO with a constrained optional field orpartial=Truefailed to build.The meta now goes on the single constrainable member of the union. If a union has more than one member that could take constraints there is nowhere obvious to put it, so it stays on the union as before. Partial DTOs no longer need to drop
min_lengthandmax_length, which was a workaround for the same problem.The old workaround test is replaced by a parametrized one covering each constraint on both backends, and I added tests for
Union[Annotated[str, Meta], UnsetType]andOptional[Annotated[str, Meta]]. They are in tests/unit/test_dto/test_factory/test_integration.py and fail without the change.Closes #4181
📚 Documentation preview 📚: https://litestar-org.github.io/litestar-docs-preview/4988