Custom Type Json schema results in KeyError: #12042

zolyfarkas-fb · 2025-07-07T18:02:11Z

zolyfarkas-fb
Jul 7, 2025

I have the following simple model:

from typing import Annotated

from pydantic import BaseModel, PlainValidator, WithJsonSchema


class TestPydanticModelNode(BaseModel):
    name: str
    next: 'TestPydanticModelNode | None' = None


class TestPydanticModelList(BaseModel):
    name: str
    head: Annotated[
        TestPydanticModelNode,
        PlainValidator(lambda x: x),
        WithJsonSchema(TestPydanticModelNode.model_json_schema()),
    ]

the following test:

    def test_recurrsive_pydantic(self) -> None:
        print(TestPydanticModelList.model_json_schema())

fails with:

...
  File "pydantic/json_schema.py", line 2184, in get_schema_from_definitions
    def_ref = self.json_to_defs_refs[json_ref]
              ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: '#/$defs/TestPydanticModelNode'

this is a simplified version of a custom type that I have that is designed to reproduce the issue I am facing.

Is this a bug? or am I doing something wrong?

thanks!

Viicos · 2025-07-10T08:50:35Z

Viicos
Jul 10, 2025
Maintainer

Pydantic sets expectations when trying to generate a JSON Schema, related to references. If you look at the JSON Schema of TestPydanticModelNode, you will see that it is defined in the definitions, and the referenced in next.

Using WithJsonSchema, the definition and reference will be set arbitrarily in the JSON Schema, and not properly collected by Pydantic to set it in a top level definitions mapping for TestPydanticModelList.

With your current example, setting WithJsonSchema(TestPydanticModelNode.model_json_schema()) seems unnecessary, so I'll need more details about your exact use case.

2 replies

zolyfarkas-fb Aug 5, 2025
Author

In my case TestPydanticModelNode is not a pydantic model, it is a generated class (protobuf, thrift) that uses a custom json serializer like:

class TestPydanticModelNode:
   ...

class TestPydanticModelList(BaseModel):
    name: str
    head: Annotated[
        TestPydanticModelNode,
        PlainValidator(lambda x: x),
        WithJsonSchema(introspect_json_schema(TestPydanticModelNodeGeneratedProtoBuf)),
    ]

In short I am trying to compose pydantic objects with non pydantic objects(protobuf, thrift, avro, etc..)
the error encountered is the same as in my original example.

Viicos Aug 5, 2025
Maintainer

I won't be of great help without an MRE of the issue. Could you please provide one?

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Viicos Jul 10, 2025 Maintainer

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zolyfarkas-fb Aug 5, 2025 Author

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Viicos Aug 5, 2025 Maintainer

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Jul 7, 2025

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Jul 10, 2025
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zolyfarkas-fb Aug 5, 2025
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