Pydantic - How to differentiate between types that look the same

showcase.py

from typing import Literal
from pydantic import BaseModel

class SomethingThatHasAName(BaseModel):
    name: str

class User(SomethingThatHasAName):
    ...
class Product(SomethingThatHasAName):
    ...

class Document(BaseModel):
    relation: User | Product


class UserFixed(SomethingThatHasAName):
    type_name: Literal["User"] = "User"

class ProductFixed(SomethingThatHasAName):
    type_name: Literal["Product"] = "Product"

class DocumentFixed(BaseModel):
    relation: UserFixed | ProductFixed




if __name__ == "__main__":
    doc = Document(relation=Product(name="i am a product"))
    # Check that the type is Product
    assert isinstance(doc.relation, Product)
    # serialize
    serialized_str = doc.model_dump_json()
    # Read it back
    doc2 = Document.model_validate_json(serialized_str)
    # Check that the type is Product
    assert isinstance(doc2.relation, Product), "Expected Product, got User" # Fails


    # Fixed version
    doc_fixed = DocumentFixed(relation=ProductFixed(name="i am a product"))
    # Check that the type is Product
    assert isinstance(doc_fixed.relation, ProductFixed)
    # serialize
    serialized_str_2 = doc_fixed.model_dump_json()
    # Read it back
    doc_fixed2 = DocumentFixed.model_validate_json(serialized_str_2)
    # Check that the type is Product
    assert isinstance(doc_fixed2.relation, ProductFixed), "Expected Product, got User" # Passes