Modify cart items

src/allmende_payment_system/models.py

@@ -94,6 +94,8 @@ class Product(Base):
 
     price: Mapped[decimal.Decimal] = mapped_column(Numeric(10, 2))
     unit_of_measure: Mapped[UnitsOfMeasure] = mapped_column(nullable=False)
+    allow_fractional: Mapped[bool] = mapped_column(nullable=False, default=True)
+
     # TODO: limit this to actually used vat rates?
     vat_rate: Mapped[decimal.Decimal] = mapped_column(Numeric(10, 2))
 
@@ -162,6 +164,13 @@ class OrderItem(Base):
         Numeric(10, 2), nullable=False
     )
 
+    def update_quantity(self, new_quantity: decimal.Decimal):
+        if new_quantity <= 0:
+            raise ValueError("Quantity must be positive.")
+
+        self.quantity = new_quantity
+        self.total_amount = self.product.price * new_quantity
+
 
 TransactionTypes = typing.Literal[
     "order",