fix: subscription price change detection (Issue 150)

backend/migrations/create_price_history.sql

@@ -0,0 +1,36 @@
+-- Create subscription_price_history table
+CREATE TABLE IF NOT EXISTS subscription_price_history (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  subscription_id UUID NOT NULL REFERENCES subscriptions(id) ON DELETE CASCADE,
+  old_price DECIMAL(10, 2) NOT NULL,
+  new_price DECIMAL(10, 2) NOT NULL,
+  changed_at TIMESTAMP WITH TIME ZONE DEFAULT timezone('utc'::text, now()) NOT NULL,
+  user_id UUID NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE
+);
+
+-- Enable RLS
+ALTER TABLE subscription_price_history ENABLE ROW LEVEL SECURITY;
+
+-- RLS Policies
+CREATE POLICY "Users can view their own price history"
+  ON subscription_price_history FOR SELECT
+  USING (auth.uid() = user_id);
+
+-- Function to handle price changes
+CREATE OR REPLACE FUNCTION handle_subscription_price_change()
+RETURNS TRIGGER AS $$
+BEGIN
+  IF (OLD.price IS DISTINCT FROM NEW.price) THEN
+    INSERT INTO subscription_price_history (subscription_id, old_price, new_price, user_id)
+    VALUES (NEW.id, OLD.price, NEW.price, NEW.user_id);
+  END IF;
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql SECURITY DEFINER;
+
+-- Trigger for price changes
+DROP TRIGGER IF EXISTS on_subscription_price_change ON subscriptions;
+CREATE TRIGGER on_subscription_price_change
+  AFTER UPDATE ON subscriptions
+  FOR EACH ROW
+  EXECUTE FUNCTION handle_subscription_price_change();

backend/src/routes/subscriptions.ts

@@ -109,6 +109,30 @@ router.get("/:id", validateSubscriptionOwnership, async (req: AuthenticatedReque
   }
 });
 
+/**
+ * GET /api/subscriptions/:id/price-history
+ * Get price history for a subscription
+ */
+router.get("/:id/price-history", validateSubscriptionOwnership, async (req: AuthenticatedRequest, res: Response) => {
+  try {
+    const history = await subscriptionService.getPriceHistory(
+      req.user!.id,
+      Array.isArray(req.params.id) ? req.params.id[0] : req.params.id
+    );
+
+    res.json({
+      success: true,
+      data: history,
+    });
+  } catch (error) {
+    logger.error("Get price history error:", error);
+    res.status(500).json({
+      success: false,
+      error: error instanceof Error ? error.message : "Failed to get price history",
+    });
+  }
+});
+
 /**
  * POST /api/subscriptions
  * Create new subscription with idempotency support

backend/src/services/subscription-service.ts

@@ -327,8 +327,25 @@ export class SubscriptionService {
       throw new Error(`Failed to fetch subscriptions: ${error.message}`);
     }
 
+    // Fetch latest price change for each subscription
+    const enhancedSubscriptions = await Promise.all(
+      (subscriptions || []).map(async (sub) => {
+        const { data: priceHistory } = await supabase
+          .from("subscription_price_history")
+          .select("*")
+          .eq("subscription_id", sub.id)
+          .order("changed_at", { ascending: false })
+          .limit(1);
+
+        return {
+          ...sub,
+          latest_price_change: priceHistory && priceHistory.length > 0 ? priceHistory[0] : null,
+        };
+      })
+    );
+
     return {
-      subscriptions: subscriptions || [],
+      subscriptions: enhancedSubscriptions,
       total: count || 0,
     };
   }
@@ -442,6 +459,28 @@ export class SubscriptionService {
       throw error;
     }
   }
+
+  /**
+   * Get price history for a subscription
+   */
+  async getPriceHistory(
+    userId: string,
+    subscriptionId: string
+  ): Promise<any[]> {
+    const { data, error } = await supabase
+      .from("subscription_price_history")
+      .select("*")
+      .eq("subscription_id", subscriptionId)
+      .eq("user_id", userId)
+      .order("changed_at", { ascending: false });
+
+    if (error) {
+      logger.error("Failed to fetch price history:", error);
+      throw new Error(`Failed to fetch price history: ${error.message}`);
+    }
+
+    return data || [];
+  }
 }
 
 export const subscriptionService = new SubscriptionService();

client/components/pages/subscriptions.tsx

@@ -390,6 +390,15 @@ function SubscriptionCard({
                 Unused {unusedInfo.daysSinceLastUse}d
               </span>
             )}
+            {sub.latest_price_change && (
+              <span className={`text-xs px-2 py-0.5 rounded-full font-semibold flex items-center gap-1 ${
+                sub.latest_price_change.new_price > sub.latest_price_change.old_price 
+                  ? "bg-red-100 text-red-700" 
+                  : "bg-green-100 text-green-700"
+              }`}>
+                {sub.latest_price_change.new_price > sub.latest_price_change.old_price ? "↑" : "↓"} Price Changed
+              </span>
+            )}
           </div>
           <div className="flex items-center gap-2">
             <p className={`text-xs ${darkMode ? "text-gray-400" : "text-gray-500"}`}>{sub.category}</p>