Fields with BaseModel type that contain an instance of a model that inherits from BaseModel do not get serialized correctly

[dmach]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

When I define a field with BaseModel type and assign an instance of a model that inherits from BaseModel, the value gets serialized as {}.

Actual result from the reproducer:

{'parent': {'data': 'DEFAULT'}, 'parent_base_model': {}}

Expected result:

{'parent': {'data': 'DEFAULT'}, 'parent_base_model': {'data': 'DEFAULT'}}

or a validation error while instantiating Child or setting parent_base_model field. I tried setting various model_config options and they did not have any impact (incl. validate_assignment=True and c.parent_base_model=p)

Example Code

from pydantic import *


class Parent(BaseModel):
    data: str = Field(default="DEFAULT")


class Child(BaseModel):
    parent: Parent | None = Field(default=None)
    parent_base_model: BaseModel | None = Field(default=None)


p = Parent()
c = Child(parent=p, parent_base_model=p)
print(c.dict())

Python, Pydantic & OS Version

pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=false
                 install path: /usr/lib/python3.11/site-packages/pydantic
               python version: 3.11.9 (main, Apr 08 2024, 06:18:15) [GCC]
                     platform: Linux-6.8.7-1-default-x86_64-with-glibc2.39
             related packages: email-validator-2.0.0.post2 mypy-1.9.0 typing_extensions-4.11.0
                       commit: unknown

[RajatRajdeep]

The parent_base_model attribute is of type BaseModel, and since BaseModel does not include a data attribute, the data attribute was ignored in the validation. You can refer to the documentation on extra fields for more information: link.

[dmach]

If this use case is related to the extra fields behavior, then it's not obvious for me.

Did you mean that if I call Child(parent_base_model=p) with an instance of a subclass, only the fields of the predecessor are used during serialization and fields from the subclass are implicitly "extra" fields from this point of view?

Also, setting extra="forbid" and strict=True makes no difference.

from pydantic import *


class Parent(BaseModel):
    model_config = ConfigDict(
        extra="forbid",
        strict=True,
    )

    data: str = Field(default="DEFAULT")


class Child(BaseModel):
    model_config = ConfigDict(
        extra="forbid",
        strict=True,
    )

    parent: Parent | None = Field(default=None)
    parent_base_model: BaseModel | None = Field(default=None)


p = Parent()
c = Child(parent=p, parent_base_model=p)
print(c.dict())

[sydney-runkle]

Hi @dmach,

You'll want to use the SerializeAsAny annotation if you want all of the data to be dumped for that second field. For example:

from pydantic import BaseModel, Field, SerializeAsAny


class Parent(BaseModel):
    data: str = Field(default="DEFAULT")


class Child(BaseModel):
    parent: Parent | None = Field(default=None)
    parent_base_model: SerializeAsAny[BaseModel] | None = Field(default=None)


p = Parent()
c = Child(parent=p, parent_base_model=p)
print(c.model_dump())
#> {'parent': {'data': 'DEFAULT'}, 'parent_base_model': {'data': 'DEFAULT'}}

Ping me if you have any questions. You can learn more about that annotation here!