Initial release

backend/app/services/draw_service.py

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+import random
+import uuid
+from datetime import datetime
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.lottery import Lottery, LotteryStatus
+from app.models.participant import Participant
+from app.models.prize import Prize
+from app.models.user import User
+from app.models.winner import Winner
+
+
+async def execute_draw(db: AsyncSession, lottery_id: uuid.UUID, trigger: str = "manual") -> list[dict]:
+    """
+    Draw winners by randomly pairing shuffled participants with shuffled prizes.
+    Both lists are shuffled independently so prize order has no bearing on results.
+    n_winners = min(participants, total prize slots).
+
+    Uses SELECT FOR UPDATE to prevent concurrent draws from producing duplicate
+    winner sets. The status is transitioned to DRAWING inside the lock before
+    any reads, so a second concurrent request will fail the status check.
+    """
+    # Lock the lottery row - serialises all concurrent draw requests
+    lottery_result = await db.execute(
+        select(Lottery).where(Lottery.id == lottery_id).with_for_update()
+    )
+    lottery = lottery_result.scalar_one()
+
+    # Re-check status under lock; may have changed since the router pre-checked
+    if lottery.status != LotteryStatus.ACTIVE:
+        raise ValueError("Lottery is no longer active")
+
+    # Mark as DRAWING immediately so any concurrent request sees the changed
+    # status and aborts before we even touch participants/prizes
+    lottery.status = LotteryStatus.DRAWING
+    await db.flush()
+
+    # Load participants with user info
+    result = await db.execute(
+        select(Participant, User)
+        .join(User, Participant.user_id == User.id)
+        .where(Participant.lottery_id == lottery_id)
+    )
+    rows = result.all()
+
+    if not rows:
+        # Roll back the DRAWING status so the lottery stays usable
+        lottery.status = LotteryStatus.ACTIVE
+        await db.commit()
+        raise ValueError("No participants in this lottery")
+
+    # Load prizes
+    prize_result = await db.execute(
+        select(Prize).where(Prize.lottery_id == lottery_id)
+    )
+    prizes = prize_result.scalars().all()
+
+    if not prizes:
+        lottery.status = LotteryStatus.ACTIVE
+        await db.commit()
+        raise ValueError("No prizes configured")
+
+    # Expand prizes by quantity: Prize(qty=3) → [Prize, Prize, Prize]
+    expanded_prizes: list[Prize] = []
+    for prize in prizes:
+        expanded_prizes.extend([prize] * prize.quantity)
+
+    random.shuffle(expanded_prizes)
+    now = datetime.utcnow()
+
+    winners_data = []
+    if lottery.allow_multiple_wins:
+        # Draw with replacement: all prize slots awarded, same participant can win multiple
+        for prize in expanded_prizes:
+            participant, user = random.choice(rows)
+            db.add(Winner(
+                lottery_id=lottery_id,
+                user_id=user.id,
+                prize_id=prize.id,
+                prize_name=prize.name,
+                drawn_at=now,
+            ))
+            winners_data.append({
+                "user_id": user.id,
+                "username": user.username,
+                "first_name": user.first_name,
+                "last_name": user.last_name,
+                "prize_name": prize.name,
+                "drawn_at": now,
+            })
+    else:
+        # No repeat wins: shuffle participants and pair 1-to-1 with prizes
+        random.shuffle(rows)
+        n_winners = min(len(rows), len(expanded_prizes))
+        for i in range(n_winners):
+            participant, user = rows[i]
+            prize = expanded_prizes[i]
+            db.add(Winner(
+                lottery_id=lottery_id,
+                user_id=user.id,
+                prize_id=prize.id,
+                prize_name=prize.name,
+                drawn_at=now,
+            ))
+            winners_data.append({
+                "user_id": user.id,
+                "username": user.username,
+                "first_name": user.first_name,
+                "last_name": user.last_name,
+                "prize_name": prize.name,
+                "drawn_at": now,
+            })
+
+    lottery.status = LotteryStatus.FINISHED
+    lottery.drawn_at = now
+    lottery.draw_trigger = trigger
+
+    await db.commit()
+    return winners_data