Given an already-loaded dictionary customers:

class Customer(BaseModel):
   id: str
   first_name: str
   last_name: str
   address: Address

customers: dict[str, Customer] = [customer.id: customer for customer in load_customers()]

The Invoice depends on the customer:

class Invoice(BaseModel):
   order: list[Item]
   amount: float
   customer_id: str

   @property
   def customer(self) -> Customer:
     return customers[self.customer_id]

Is it possible to resolve the customer_id immediately on parsing the object?

class Invoice(BaseModel):
   order: list[Item]
   amount: float
   customer: Customer

   @field_validator('customer_id')
   @classmethod
   def parse_customer(cls, v: str, info) -> Customer:
      return customers[v]