What are the key differences between Pydantic v1 and v2, and how does FastAPI use Pydantic v2?

from pydantic import BaseModel, field_validator, model_validator, ConfigDict
from typing import Self

class OrderItem(BaseModel):
    product_id: int
    quantity:   int
    unit_price: float

    @field_validator("quantity")
    @classmethod
    def quantity_positive(cls, v: int) -> int:
        if v <= 0:
            raise ValueError("quantity must be positive")
        return v

class Order(BaseModel):
    items: list[OrderItem]
    discount: float = 0.0

    model_config = ConfigDict(from_attributes=True)  # replaces orm_mode

    @model_validator(mode="after")
    def discount_valid(self) -> Self:
        if self.discount < 0 or self.discount > 1:
            raise ValueError("discount must be between 0 and 1")
        return self

    @property
    def total(self) -> float:
        subtotal = sum(i.quantity * i.unit_price for i in self.items)
        return round(subtotal * (1 - self.discount), 2)

# Serialisation
order = Order(items=[OrderItem(product_id=1, quantity=2, unit_price=9.99)])
print(order.model_dump())        # dict
print(order.model_dump_json())   # JSON bytes
print(order.model_json_schema()) # JSON Schema