feat: add transactions API with holdings update

backend/app/api/portfolio.py

@@ -8,11 +8,12 @@ from sqlalchemy.orm import Session
 
 from app.core.database import get_db
 from app.api.deps import CurrentUser
-from app.models.portfolio import Portfolio, PortfolioType, Target, Holding
+from app.models.portfolio import Portfolio, PortfolioType, Target, Holding, Transaction, TransactionType
 from app.schemas.portfolio import (
     PortfolioCreate, PortfolioUpdate, PortfolioResponse, PortfolioDetail,
     TargetCreate, TargetResponse,
     HoldingCreate, HoldingResponse,
+    TransactionCreate, TransactionResponse,
 )
 
 router = APIRouter(prefix="/api/portfolios", tags=["portfolios"])
@@ -208,3 +209,83 @@ async def set_holdings(
 
     db.commit()
     return new_holdings
+
+
+@router.get("/{portfolio_id}/transactions", response_model=List[TransactionResponse])
+async def get_transactions(
+    portfolio_id: int,
+    current_user: CurrentUser,
+    db: Session = Depends(get_db),
+    limit: int = 50,
+):
+    """Get transaction history for a portfolio."""
+    _get_portfolio(db, portfolio_id, current_user.id)
+    transactions = (
+        db.query(Transaction)
+        .filter(Transaction.portfolio_id == portfolio_id)
+        .order_by(Transaction.executed_at.desc())
+        .limit(limit)
+        .all()
+    )
+    return transactions
+
+
+@router.post("/{portfolio_id}/transactions", response_model=TransactionResponse, status_code=status.HTTP_201_CREATED)
+async def add_transaction(
+    portfolio_id: int,
+    data: TransactionCreate,
+    current_user: CurrentUser,
+    db: Session = Depends(get_db),
+):
+    """Add a transaction and update holdings accordingly."""
+    _get_portfolio(db, portfolio_id, current_user.id)
+
+    tx_type = TransactionType(data.tx_type)
+
+    # Create transaction
+    transaction = Transaction(
+        portfolio_id=portfolio_id,
+        ticker=data.ticker,
+        tx_type=tx_type,
+        quantity=data.quantity,
+        price=data.price,
+        executed_at=data.executed_at,
+        memo=data.memo,
+    )
+    db.add(transaction)
+
+    # Update holding
+    holding = db.query(Holding).filter(
+        Holding.portfolio_id == portfolio_id,
+        Holding.ticker == data.ticker,
+    ).first()
+
+    if tx_type == TransactionType.BUY:
+        if holding:
+            # Update average price
+            total_value = (holding.quantity * holding.avg_price) + (data.quantity * data.price)
+            new_quantity = holding.quantity + data.quantity
+            holding.quantity = new_quantity
+            holding.avg_price = total_value / new_quantity if new_quantity > 0 else 0
+        else:
+            # Create new holding
+            holding = Holding(
+                portfolio_id=portfolio_id,
+                ticker=data.ticker,
+                quantity=data.quantity,
+                avg_price=data.price,
+            )
+            db.add(holding)
+    elif tx_type == TransactionType.SELL:
+        if not holding or holding.quantity < data.quantity:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Insufficient quantity for {data.ticker}"
+            )
+        holding.quantity -= data.quantity
+        if holding.quantity == 0:
+            db.delete(holding)
+
+    db.commit()
+    db.refresh(transaction)
+    return transaction