0 is out of stock and nothing is available from now on

src/models/product_variants.rs

@@ -45,6 +45,30 @@ impl Model {
     pub fn business_on_sale(&self) -> bool {
         matches!(self.business_sale_price_cents, Some(sale) if sale < self.price_cents)
     }
+
+    /// Whether the variant's inventory is tracked. A `None` stock means
+    /// "available, not tracked" (always purchasable, unlimited).
+    #[must_use]
+    pub fn tracked(&self) -> bool {
+        self.stock.is_some()
+    }
+
+    /// Whether the variant can currently be bought: untracked variants are always
+    /// available; tracked ones need a positive quantity on hand.
+    #[must_use]
+    pub fn in_stock(&self) -> bool {
+        self.stock.map_or(true, |s| s > 0)
+    }
+
+    /// Clamp a desired quantity to what's available: capped at the tracked stock,
+    /// or left as-is (only floored at 0) when untracked.
+    #[must_use]
+    pub fn cap(&self, qty: i32) -> i32 {
+        match self.stock {
+            Some(s) => qty.clamp(0, s),
+            None => qty.max(0),
+        }
+    }
 }
 
 // implement your write-oriented logic here