🐛 Fix Field alias support for Pydantic v2 with backward compatibility #1577

ravishan16 · 2025-09-25T22:56:03Z

Description

This PR fixes the Field(alias="...") functionality that was broken in Pydantic v2 while maintaining full backward compatibility with Pydantic v1.

Problem

In Pydantic v2, the alias parameter in SQLModel's Field function was not working correctly for validation and serialization. The issue was that Pydantic v2 requires explicit validation_alias and serialization_alias parameters, but SQLModel wasn't automatically propagating the alias value to these parameters.

Solution

Changes Made:

Enhanced Field function:
- Added validation_alias and serialization_alias parameters
- Implemented version-specific parameter handling
Automatic alias propagation:
- Modified post_init_field_info in _compat.py to automatically set validation_alias and serialization_alias from alias when not explicitly provided (Pydantic v2 only)
Backward compatibility:
- Filter out v2-only parameters from FieldInfo in Pydantic v1
- Raise clear RuntimeError when v2 features are used with v1
- Conditional test execution based on Pydantic version
Comprehensive testing:
- Added extensive test coverage for both Pydantic v1 and v2
- Tests automatically skip v2-specific features when running with v1
- Covers all alias scenarios including edge cases

Breaking Changes
None. This is purely a bug fix with backward compatibility maintained.

Closes
Fixes #1536

This commit fixes Field(alias="...") functionality in Pydantic v2 while maintaining full backward compatibility with Pydantic v1.

YuriiMotov

The idea looks good to me!

A couple of more things:

I think it would be nice to use alias as a default value for DB column. It can then be overridden by sa_column=Column("column_name")
We should probably test how it works with alias generators specified in model's config (is it supported? - UPD: YES)

PS: @svlandeg, sorry, I only noticed you self-assigned this PR when I was about to press "Submit review".. Hope you don't mind I submitted this review)

YuriiMotov · 2025-09-30T17:59:10Z

sqlmodel/_compat.py

    def post_init_field_info(field_info: FieldInfo) -> None:
-        return None
+        if IS_PYDANTIC_V2:


This function is declared inside the if IS_PYDANTIC_V2: block. So, this condition looks not necessary here.
Am I missing something?

Added for backward compatibility with Pydantic v1.
Without this, Pydantic tests v1 would fail.

I just checked it with Python 3.11 and Pydantic 1.10.22 - tests passed.
Could you please check it one more time?

def post_init_field_info(field_info: FieldInfo) -> None: if field_info.alias and not field_info.validation_alias: field_info.validation_alias = field_info.alias if field_info.alias and not field_info.serialization_alias: field_info.serialization_alias = field_info.alias

sqlmodel/main.py

YuriiMotov · 2025-09-30T18:11:37Z

tests/test_aliases.py

+# Models
+
+
+class PydanticUser(BaseModel):


We should think whether we want to keep parameterizing tests with Pydantic models.
Initially I parametrized tests with both, Pydantic and SQLModel models to show the difference in behavior between SQLModel and Pydantic models.

If it's useful to keep this parametrization in repo? 🤔
Tests would be a bit simpler without it, but on the other hand this way we ensure it works the same as with Pydantic model

Personally, I think it’s useful to keep the parametrization for now, since it acts as a kind of integration test between the two libraries.

tests/test_aliases.py

YuriiMotov · 2025-09-30T18:26:07Z

sqlmodel/main.py

    *,
    default_factory: Optional[NoArgAnyCallable] = None,
    alias: Optional[str] = None,
+    validation_alias: Optional[str] = None,


Pydantic's validation_alias type annotation is wider (validation_alias: str | AliasPath | AliasChoices | None), but I think it's fine if we only support str for now and extend it later

I don't have a strong preference on this.

Co-authored-by: Motov Yurii <[email protected]>

…Yurii's comment https://github.com/fastapi/sqlmodel/pull/1577/files/34d2a4500e26a62f7b9e30982eebd44538e08859\#r2392422553

svlandeg · 2025-10-01T09:37:29Z

Hope you don't mind I submitted this review

Of course not! I tentatively assign PRs when I want to review them in the near future, just to keep track of them, but sometimes other tasks come in between 🙈 Feel free to review whatever you want!

ravishan16 · 2025-10-04T20:15:43Z

@YuriiMotov @svlandeg I’ve addressed all the comments from the review. Let me know if there’s anything else you’d like me to focus on.

YuriiMotov

Good! We are almost there)

I added a few comments.

Tested it a bit more an found a use case that is broken with this PR applied.
This is the workaround people use for now. Although it's not documented and it's a misuse of schema_extra, I think it would be nice to support passing validation_alias and serialization_alias via schema_extra, and show a warning message in such cases.

from sqlmodel import SQLModel, Field

class MyModel(SQLModel):
    f: str = Field(schema_extra={"validation_alias": "f_alias"})

m = MyModel.model_validate({"f_alias": "asd"})
assert m.f == "asd"

Also, we may want to add more tests (just ideas):

alias + validation_alias = validation_alias takes precedence
alias + serialization_alias = serialization_alias takes precedence
alias_generator in model config
alias_generator in model config + alias for field = alias takes precedence
passing validation_alias and serialization_alias via schema_extra

YuriiMotov · 2025-10-06T08:40:39Z

tests/test_aliases.py

+def test_serialization_alias_runtimeerror_pydantic_v1():
+    if VERSION.startswith("2."):
+        pytest.skip("Only relevant for Pydantic v1")


Suggested change

def test_serialization_alias_runtimeerror_pydantic_v1():

if VERSION.startswith("2."):

pytest.skip("Only relevant for Pydantic v1")

@needs_pydanticv1

def test_serialization_alias_runtimeerror_pydantic_v1():

YuriiMotov · 2025-10-06T08:41:19Z

tests/test_aliases.py

+def test_validation_alias_runtimeerror_pydantic_v1():
+    if VERSION.startswith("2."):
+        pytest.skip("Only relevant for Pydantic v1")


Suggested change

def test_validation_alias_runtimeerror_pydantic_v1():

if VERSION.startswith("2."):

pytest.skip("Only relevant for Pydantic v1")

@needs_pydanticv1

def test_validation_alias_runtimeerror_pydantic_v1():

YuriiMotov · 2025-10-06T09:20:17Z

tests/test_aliases.py

+from pydantic import Field as PField
+from sqlmodel import Field, SQLModel
+
+from tests.conftest import needs_pydanticv2


Suggested change

from tests.conftest import needs_pydanticv2

from tests.conftest import needs_pydanticv1, needs_pydanticv2

ravishan16 added 5 commits September 25, 2025 15:53

feat: Add support for Pydantic v2 aliases

785270a

fix: Skip alias tests in Pydantic v1

72e8c36

feat: Add support for Pydantic v2 aliases

15c7954

fix: Skip alias tests in Pydantic v1

3cc2b1e

fix: Add Pydantic v2 alias support with backward compatibility

34d2a45

This commit fixes Field(alias="...") functionality in Pydantic v2 while maintaining full backward compatibility with Pydantic v1.

ravishan16 marked this pull request as ready for review September 25, 2025 23:02

ravishan16 mentioned this pull request Sep 25, 2025

Field(alias="") doesn't work with Pydantic V2 #1536

Open

1 task

svlandeg added the bug Something isn't working label Sep 26, 2025

svlandeg self-assigned this Sep 26, 2025

YuriiMotov reviewed Sep 30, 2025

View reviewed changes

ravishan16 and others added 3 commits September 30, 2025 16:49

Apply suggestion from @YuriiMotov

6667109

Co-authored-by: Motov Yurii <[email protected]>

fix: move this outside of if..else.. block to avoid duplicati as per …

382a52d

…Yurii's comment https://github.com/fastapi/sqlmodel/pull/1577/files/34d2a4500e26a62f7b9e30982eebd44538e08859\#r2392422553

fix: Addressed yurii's comments

d3e761e

svlandeg removed their assignment Oct 6, 2025

YuriiMotov reviewed Oct 6, 2025

View reviewed changes

	from tests.conftest import needs_pydanticv2
	from tests.conftest import needs_pydanticv1, needs_pydanticv2

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🐛 Fix Field alias support for Pydantic v2 with backward compatibility #1577

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ravishan16 commented Sep 25, 2025

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Changes Made:

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svlandeg commented Oct 1, 2025

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