Projection Patterns

# projection-patterns — templates and worked examples

## Templates

### Template 1: Basic Projector

```python
from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Dict, Any, Callable, List
import asyncpg

@dataclass
class Event:
    stream_id: str
    event_type: str
    data: dict
    version: int
    global_position: int


class Projection(ABC):
    """Base class for projections."""

    @property
    @abstractmethod
    def name(self) -> str:
        """Unique projection name for checkpointing."""
        pass

    @abstractmethod
    def handles(self) -> List[str]:
        """List of event types this projection handles."""
        pass

    @abstractmethod
    async def apply(self, event: Event) -> None:
        """Apply event to the read model."""
        pass


class Projector:
    """Runs projections from event store."""

    def __init__(self, event_store, checkpoint_store):
        self.event_store = event_store
        self.checkpoint_store = checkpoint_store
        self.projections: List[Projection] = []

    def register(self, projection: Projection):
        self.projections.append(projection)

    async def run(self, batch_size: int = 100):
        """Run all projections continuously."""
        while True:
            for projection in self.projections:
                await self._run_projection(projection, batch_size)
            await asyncio.sleep(0.1)

    async def _run_projection(self, projection: Projection, batch_size: int):
        checkpoint = await self.checkpoint_store.get(projection.name)
        position = checkpoint or 0

        events = await self.event_store.read_all(position, batch_size)

        for event in events:
            if event.event_type in projection.handles():
                await projection.apply(event)

            await self.checkpoint_store.save(
                projection.name,
                event.global_position
            )

    async def rebuild(self, projection: Projection):
        """Rebuild a projection from scratch."""
        await self.checkpoint_store.delete(projection.name)
        # Optionally clear read model tables
        await self._run_projection(projection, batch_size=1000)
```

### Template 2: Order Summary Projection

```python
class OrderSummaryProjection(Projection):
    """Projects order events to a summary read model."""

    def __init__(self, db_pool: asyncpg.Pool):
        self.pool = db_pool

    @property
    def name(self) -> str:
        return "order_summary"

    def handles(self) -> List[str]:
        return [
            "OrderCreated",
            "OrderItemAdded",
            "OrderItemRemoved",
            "OrderShipped",
            "OrderCompleted",
            "OrderCancelled"
        ]

    async def apply(self, event: Event) -> None:
        handlers = {
            "OrderCreated": self._handle_created,
            "OrderItemAdded": self._handle_item_added,
            "OrderItemRemoved": self._handle_item_removed,
            "OrderShipped": self._handle_shipped,
            "OrderCompleted": self._handle_completed,
            "OrderCancelled": self._handle_cancelled,
        }

        handler = handlers.get(event.event_type)
        if handler:
            await handler(event)

    async def _handle_created(self, event: Event):
        async with self.pool.acquire() as conn:
            await conn.execute(
                """
                INSERT INTO order_summaries
                (order_id, customer_id, status, total_amount, item_count, created_at)
                VALUES ($1, $2, $3, $4, $5, $6)
                """,
                event.data['order_id'],
                event.data['customer_id'],
                'pending',
                0,
                0,
                event.data['created_at']
            )

    async def _handle_item_added(self, event: Event):
        async with self.pool.acquire() as conn:
            await conn.execute(
                """