feat: make JSON serialization of referenced pydantic models possible

[PrettyWood]

Change Summary

Allow to customize serialization of fields with BaseModel type

from typing import List, Optional

from pydantic import BaseModel


class Address(BaseModel):
    city: str
    country: str


class User(BaseModel):
    name: str
    address: Address
    friends: Optional[List['User']] = None

    class Config:
        json_encoders = {
            Address: lambda a: f'{a.city} ({a.country})',
            'User': lambda u: f'{u.name} in {u.address.city} '
                              f'({u.address.country[:2].upper()})',
        }


User.update_forward_refs()

wolfgang = User(
    name='Wolfgang',
    address=Address(city='Berlin', country='Deutschland'),
    friends=[
        User(name='Pierre', address=Address(city='Paris', country='France')),
        User(name='John', address=Address(city='London', country='UK')),
    ],
)
print(wolfgang.json())
# {"name": "Wolfgang", "address": "Berlin (Deutschland)", "friends": ["Pierre in Paris (FR)", "John in London (UK)"]}

Related issue number

#2622

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[samuelcolvin]

Seems like a good start.

[samuelcolvin]

please update.

[PrettyWood]

Thanks for the remarks. Please review the updated version

[mcauto]

When is it merging?

[samuelcolvin]

thank you.