┌─────────────────────────────────────────────────────────┐
│                 ORDER AGGREGATE                         │
│  ┌─────────────────────────────────────────────────┐   │
│  │         Order (Aggregate Root)                   │   │
│  │  • id: UUID (UUIDv7)                            │   │
│  │  • customer_id: UUID (reference by ID!)         │   │
│  │  • status: OrderStatus                          │   │
│  └─────────────────────────────────────────────────┘   │
│           │                      │                      │
│  ┌────────────────┐    ┌────────────────┐              │
│  │  OrderItem     │    │  OrderItem     │              │
│  │  (child)       │    │  (child)       │              │
│  └────────────────┘    └────────────────┘              │
│                                                         │
│  INVARIANTS enforced by root:                          │
│  • Total = sum of items                                │
│  • Max 100 items per order                             │
│  • Cannot modify after shipped                         │
└─────────────────────────────────────────────────────────┘

from dataclasses import dataclass, field
from uuid import UUID
from uuid_utils import uuid7

@dataclass
class OrderAggregate:
    """Aggregate root with invariant enforcement."""

    id: UUID = field(default_factory=uuid7)
    customer_id: UUID  # Reference by ID, not Customer object!
    _items: list["OrderItem"] = field(default_factory=list)
    status: str = "draft"

    MAX_ITEMS = 100

    def add_item(self, product_id: UUID, quantity: int, price: Money) -> None:
        """Add item with invariant checks."""
        self._ensure_modifiable()
        if len(self._items) >= self.MAX_ITEMS:
            raise DomainError("Max items exceeded")
        self._items.append(OrderItem(product_id, quantity, price))

    def _ensure_modifiable(self) -> None:
        if self.status != "draft":
            raise DomainError(f"Cannot modify {self.status} order")

# NEVER reference aggregates by object
customer: Customer  # WRONG → customer_id: UUID

# NEVER modify multiple aggregates in one transaction
order.submit()
inventory.reserve(items)  # WRONG - use domain events

# NEVER expose mutable collections
def items(self) -> list:
    return self._items  # WRONG → return tuple(self._items)

# NEVER have unbounded collections
orders: list[Order]  # WRONG - grows unbounded