diff --git a/pages/api/cards/[id]/ownership.js b/pages/api/cards/[id]/ownership.js
index 0b4c84c..cf1405e 100644
--- a/pages/api/cards/[id]/ownership.js
+++ b/pages/api/cards/[id]/ownership.js
@@ -2,7 +2,19 @@ import { sql } from '@vercel/postgres';
 import { getUserFromRequest } from '../../../../lib/permission-middleware';
 
 export default async function handler(req, res) {
-  const { id } = req.query;
+  // Set CORS headers
+  res.setHeader('Access-Control-Allow-Origin', '*');
+  res.setHeader('Access-Control-Allow-Methods', 'POST, OPTIONS');
+  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
+
+  if (req.method === 'OPTIONS') {
+    res.status(200).end();
+    return;
+  }
+
+  if (req.method !== 'POST') {
+    return res.status(405).json({ error: 'Method not allowed' });
+  }
 
   try {
     // Get authenticated user
@@ -11,75 +23,95 @@ export default async function handler(req, res) {
       return res.status(401).json({ error: 'Authentication required' });
     }
 
-    if (req.method === 'GET') {
-      // Get user's ownership of this card
+    const { id } = req.query;
+    const { quantity } = req.body;
+
+    if (!id || quantity === undefined) {
+      return res.status(400).json({ error: 'Card ID and quantity are required' });
+    }
+
+    const cardId = parseInt(id);
+    const cardQuantity = parseInt(quantity);
+
+    if (isNaN(cardId) || isNaN(cardQuantity) || cardQuantity < 0) {
+      return res.status(400).json({ error: 'Invalid card ID or quantity' });
+    }
+
+    // Verify the card exists
+    const cardCheck = await sql`SELECT id, name FROM cards WHERE id = ${cardId}`;
+    if (cardCheck.rows.length === 0) {
+      return res.status(404).json({ error: 'Card not found' });
+    }
+
+    const card = cardCheck.rows[0];
+
+    // Find the user's "All My Cards" collection
+    const allMyCardsCollection = await sql`
+      SELECT id FROM collections 
+      WHERE user_id = ${user.userId} 
+      AND name = 'All My Cards' 
+      AND is_system_collection = true
+    `;
+
+    if (allMyCardsCollection.rows.length === 0) {
+      return res.status(500).json({ error: 'All My Cards collection not found' });
+    }
+
+    const collectionId = allMyCardsCollection.rows[0].id;
+
+    if (cardQuantity > 0) {
+      // Insert or update user's card ownership
       const result = await sql`
-        SELECT uc.quantity
-        FROM user_cards uc
-        WHERE uc.user_id = ${user.userId} AND uc.card_id = ${id}
+        INSERT INTO user_cards (user_id, card_id, quantity, created_at, updated_at)
+        VALUES (${user.userId}, ${cardId}, ${cardQuantity}, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+        ON CONFLICT (user_id, card_id)
+        DO UPDATE SET 
+          quantity = ${cardQuantity},
+          updated_at = CURRENT_TIMESTAMP
+        RETURNING *
       `;
 
-      const quantity = result.rows.length > 0 ? result.rows[0].quantity : 0;
+      // Sync with "All My Cards" collection
+      const collectionCardResult = await sql`
+        INSERT INTO collection_cards (collection_id, card_id, quantity, created_at, updated_at)
+        VALUES (${collectionId}, ${cardId}, ${cardQuantity}, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+        ON CONFLICT (collection_id, card_id)
+        DO UPDATE SET 
+          quantity = ${cardQuantity},
+          updated_at = CURRENT_TIMESTAMP
+        RETURNING *
+      `;
 
       res.status(200).json({
         success: true,
-        quantity: quantity
+        message: 'Card ownership updated and synced to All My Cards collection',
+        card: {
+          id: card.id,
+          name: card.name,
+          quantity: result.rows[0].quantity
+        }
       });
-    } else if (req.method === 'POST') {
-      const { quantity } = req.body;
 
-      // Check if card exists
-      const cardCheck = await sql`
-        SELECT id, name FROM cards WHERE id = ${id}
-      `;
-
-      if (cardCheck.rows.length === 0) {
-        return res.status(404).json({ error: 'Card not found' });
-      }
-
-      const card = cardCheck.rows[0];
-
-      if (quantity > 0) {
-        // Insert or update user's card ownership
-        const result = await sql`
-          INSERT INTO user_cards (user_id, card_id, quantity)
-          VALUES (${user.userId}, ${id}, ${quantity})
-          ON CONFLICT (user_id, card_id)
-          DO UPDATE SET 
-            quantity = ${quantity},
-            updated_at = CURRENT_TIMESTAMP
-          RETURNING *
-        `;
-
-        res.status(200).json({
-          success: true,
-          card: {
-            id: card.id,
-            name: card.name,
-            quantity: result.rows[0].quantity
-          }
-        });
-      } else {
-        // Remove card from user's collection if quantity is 0
-        await sql`
-          DELETE FROM user_cards 
-          WHERE user_id = ${user.userId} AND card_id = ${id}
-        `;
-
-        res.status(200).json({
-          success: true,
-          card: {
-            id: card.id,
-            name: card.name,
-            quantity: 0
-          }
-        });
-      }
     } else {
-      res.status(405).json({ error: 'Method not allowed' });
+      // Remove card ownership
+      await sql`DELETE FROM user_cards WHERE user_id = ${user.userId} AND card_id = ${cardId}`;
+      
+      // Remove from "All My Cards" collection
+      await sql`DELETE FROM collection_cards WHERE collection_id = ${collectionId} AND card_id = ${cardId}`;
+
+      res.status(200).json({
+        success: true,
+        message: 'Card ownership removed and synced from All My Cards collection',
+        card: {
+          id: card.id,
+          name: card.name,
+          quantity: 0
+        }
+      });
     }
+
   } catch (error) {
     console.error('Error handling ownership:', error);
-    res.status(500).json({ error: 'Failed to handle ownership' });
+    res.status(500).json({ error: 'Internal server error' });
   }
 } 
\ No newline at end of file
diff --git a/scripts/fix-user-cards-constraints.js b/scripts/fix-user-cards-constraints.js
new file mode 100644
index 0000000..425ab5f
--- /dev/null
+++ b/scripts/fix-user-cards-constraints.js
@@ -0,0 +1,168 @@
+#!/usr/bin/env node
+
+import dotenv from 'dotenv';
+import { neon } from '@neondatabase/serverless';
+
+dotenv.config({ path: '.env.local' });
+
+async function fixUserCardsConstraints() {
+  const sql = neon(process.env.POSTGRES_URL);
+
+  try {
+    console.log('🔧 Fixing user_cards table constraints...');
+
+    // First, check if there are any duplicate entries that would prevent adding the constraint
+    console.log('🔍 Checking for duplicate entries...');
+    const duplicates = await sql`
+      SELECT user_id, card_id, COUNT(*) as count
+      FROM user_cards
+      GROUP BY user_id, card_id
+      HAVING COUNT(*) > 1
+    `;
+
+    if (duplicates.length > 0) {
+      console.log(`⚠️  Found ${duplicates.length} duplicate entries. Cleaning up...`);
+      
+      // Remove duplicates by keeping the latest entry for each user/card combination
+      for (const dup of duplicates) {
+        console.log(`Cleaning duplicates for user ${dup.user_id}, card ${dup.card_id}...`);
+        
+        // Keep only the most recent entry
+        await sql`
+          DELETE FROM user_cards 
+          WHERE user_id = ${dup.user_id} AND card_id = ${dup.card_id}
+          AND id NOT IN (
+            SELECT id FROM user_cards 
+            WHERE user_id = ${dup.user_id} AND card_id = ${dup.card_id}
+            ORDER BY updated_at DESC, id DESC 
+            LIMIT 1
+          )
+        `;
+      }
+      console.log('✅ Cleaned up duplicate entries');
+    } else {
+      console.log('✅ No duplicate entries found');
+    }
+
+    // Add the unique constraint to user_cards
+    console.log('🔒 Adding unique constraint on user_cards (user_id, card_id)...');
+    try {
+      await sql`
+        ALTER TABLE user_cards 
+        ADD CONSTRAINT user_cards_user_card_unique 
+        UNIQUE (user_id, card_id)
+      `;
+      console.log('✅ Added user_cards unique constraint');
+    } catch (error) {
+      if (error.message.includes('already exists')) {
+        console.log('✅ User_cards unique constraint already exists');
+      } else {
+        throw error;
+      }
+    }
+
+    // Add the unique constraint to collection_cards
+    console.log('🔒 Adding unique constraint on collection_cards (collection_id, card_id)...');
+    try {
+      await sql`
+        ALTER TABLE collection_cards 
+        ADD CONSTRAINT collection_cards_collection_card_unique 
+        UNIQUE (collection_id, card_id)
+      `;
+      console.log('✅ Added collection_cards unique constraint');
+    } catch (error) {
+      if (error.message.includes('already exists')) {
+        console.log('✅ Collection_cards unique constraint already exists');
+      } else {
+        throw error;
+      }
+    }
+
+    // Now sync existing owned cards to "All My Cards" collections
+    console.log('🔄 Syncing owned cards to "All My Cards" collections...');
+    
+    // Get all users with owned cards
+    const usersWithOwnedCards = await sql`
+      SELECT DISTINCT uc.user_id, u.email
+      FROM user_cards uc
+      JOIN users u ON uc.user_id = u.id
+      WHERE uc.quantity > 0
+    `;
+
+    console.log(`Found ${usersWithOwnedCards.length} users with owned cards`);
+
+    for (const user of usersWithOwnedCards) {
+      try {
+        // Find the user's "All My Cards" collection
+        const allMyCardsCollection = await sql`
+          SELECT id FROM collections 
+          WHERE user_id = ${user.user_id} 
+          AND name = 'All My Cards' 
+          AND is_system_collection = true
+        `;
+
+        if (allMyCardsCollection.length === 0) {
+          console.log(`⚠️  No "All My Cards" collection found for ${user.email}`);
+          continue;
+        }
+
+        const collectionId = allMyCardsCollection[0].id;
+
+        // Get all owned cards for this user
+        const ownedCards = await sql`
+          SELECT card_id, quantity 
+          FROM user_cards 
+          WHERE user_id = ${user.user_id} AND quantity > 0
+        `;
+
+        console.log(`Syncing ${ownedCards.length} owned cards for ${user.email}...`);
+
+        // Add each owned card to the "All My Cards" collection
+        for (const ownedCard of ownedCards) {
+          // Check if card is already in the collection
+          const existingEntry = await sql`
+            SELECT id, quantity FROM collection_cards 
+            WHERE collection_id = ${collectionId} AND card_id = ${ownedCard.card_id}
+          `;
+
+          if (existingEntry.length > 0) {
+            // Update existing entry
+            await sql`
+              UPDATE collection_cards 
+              SET quantity = ${ownedCard.quantity}, updated_at = CURRENT_TIMESTAMP
+              WHERE collection_id = ${collectionId} AND card_id = ${ownedCard.card_id}
+            `;
+          } else {
+            // Insert new entry
+            await sql`
+              INSERT INTO collection_cards (collection_id, card_id, quantity, created_at, updated_at)
+              VALUES (${collectionId}, ${ownedCard.card_id}, ${ownedCard.quantity}, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+            `;
+          }
+        }
+
+        console.log(`  ✅ Synced ${ownedCards.length} cards for ${user.email}`);
+
+      } catch (error) {
+        console.error(`  ❌ Failed to sync cards for ${user.email}:`, error.message);
+      }
+    }
+
+    console.log('\n🎉 Migration completed successfully!');
+    console.log('\n📋 Summary:');
+    console.log(`   • Fixed unique constraint on user_cards table`);
+    console.log(`   • Synced owned cards to "All My Cards" collections`);
+    console.log(`   • Card ownership API should now work properly`);
+
+  } catch (error) {
+    console.error('❌ Migration failed:', error.message);
+    console.error('Full error:', error);
+    process.exit(1);
+  }
+}
+
+if (import.meta.url === `file://${process.argv[1]}`) {
+  fixUserCardsConstraints();
+}
+
+export { fixUserCardsConstraints }; 
\ No newline at end of file