Add validation to prevent booking two trades on the same day

backend/routes/exchange.py

@@ -378,6 +378,28 @@ async def create_exchange(
     slot_date = request.slot_start.date()
     validate_date_in_range(slot_date, context="book")
 
+    # Check if user already has a trade on this date
+    existing_trade_query = select(Exchange).where(
+        and_(
+            Exchange.user_id == current_user.id,
+            Exchange.slot_start >= datetime.combine(slot_date, time.min, tzinfo=UTC),
+            Exchange.slot_start
+            < datetime.combine(slot_date, time.max, tzinfo=UTC) + timedelta(days=1),
+            Exchange.status == ExchangeStatus.BOOKED,
+        )
+    )
+    existing_trade_result = await db.execute(existing_trade_query)
+    existing_trade = existing_trade_result.scalar_one_or_none()
+
+    if existing_trade:
+        raise HTTPException(
+            status_code=400,
+            detail=(
+                f"You already have a trade booked on {slot_date.strftime('%Y-%m-%d')}. "
+                f"Only one trade per day is allowed. Trade ID: {existing_trade.id}"
+            ),
+        )
+
     # Validate direction
     try:
         direction = TradeDirection(request.direction)