Book can be in multiple users' carts

payments/views.py

@@ -65,7 +65,7 @@ def success(request):
         pt = PendingTransaction(user=u, balance=transaction.price, date_transacted=timezone.now(), date_settled=one_week_in_future())
         pt.save()
         transaction.has_been_sold = True
-        transaction.cart = None
+        transaction.cart.clear()
         transaction.save()
         transaction.sold_date = datetime.now()
         sold_items.append(transaction)
@@ -95,18 +95,12 @@ def cancelled(request):
 def cart_functions(user, listing_id, fn):
     listing = get_object_or_404(ProductListing, pk=listing_id)
     if fn == "add":
-        if listing.cart is not None:
-            if listing.cart is user.cart:
-                return JsonResponse({'status': 'success', 'title': listing.textbook.title, 'seller': listing.user.email})
-            return JsonResponse({'status': "error - already in another user's cart"})
-        listing.cart = user.cart
+        listing.cart.add(user.cart)
         listing.save()       
         return JsonResponse({'status': 'success', 'title': listing.textbook.title, 'seller': listing.user.email})
     elif fn == "remove":
-        if listing.cart != user.cart:
+        if not user.cart in listing.cart.all():
             return JsonResponse({'status': "error - not authorized to perform this action"})
-        if listing.cart is None:
-            return JsonResponse({'status': "error - this item was not in the user's cart"})
-        listing.cart = None
+        listing.cart.remove(user.cart)
         listing.save()
         return JsonResponse({'status': 'success'})

textbook_exchange/migrations/0003_auto_20200427_1338.py

@@ -0,0 +1,22 @@
+# Generated by Django 3.0.3 on 2020-04-27 19:38
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('textbook_exchange', '0002_productlisting_picture_upload'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='productlisting',
+            name='cart',
+        ),
+        migrations.AddField(
+            model_name='productlisting',
+            name='cart',
+            field=models.ManyToManyField(blank=True, to='textbook_exchange.Cart'),
+        ),
+    ]

textbook_exchange/models.py

@@ -121,7 +121,7 @@ class ProductListing(models.Model):
     user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
     textbook = models.ForeignKey(Textbook, on_delete=models.CASCADE)
     class_object = models.ManyToManyField(Class) # on_delete for ManyToManyField?
-    cart = models.ForeignKey(Cart, on_delete=models.CASCADE, null=True, blank=True) # should prevent a product listing from being in multiple carts at once
+    cart = models.ManyToManyField(Cart, blank=True)
     price = models.DecimalField(max_digits=7, decimal_places=2)
     condition = models.CharField(max_length=10) # 'likenew', 'verygood', 'good', 'acceptable'
     picture_url = models.CharField(max_length=500, default="")

textbook_exchange/templates/textbook_exchange/faq.html

@@ -85,15 +85,14 @@ <h5 class="mb-0">
             <div class="card-header" id="headingFour" data-toggle="collapse" data-target="#collapseFour" aria-expanded="false" aria-controls="collapseFour">
                 <h5 class="mb-0">
                     <button class="btn btn-link collapsed">
-                    I just saw a book in the buy page! Where did it go?
+                    A book in my cart just disappeared! Where did it go?
                     </button>
                 </h5>
             </div>
             <div id="collapseFour" class="collapse" aria-labelledby="headingFour" data-parent="#accordion">
                 <div class="card-body">
-                    When someone adds a book to their cart, it is removed from the available book listings. 
-                    Don't worry though, once an item is added to someone's cart, they have 15 minutes 
-                    to finish the checkout process or it will be removed from their cart.
+                    Multiple people can have a book in their cart. Therefore, it is possible that someone else already bought
+                    it or that the seller manually marked it as sold. Try searching for other copies of the book!
                 </div>
             </div>
         </div>

textbook_exchange/views.py

@@ -215,7 +215,7 @@ def post(self, request, *args, **kwargs):
             if 'sold_listing' in self.request.POST:
                 listing_id = self.request.POST.get('sold_listing')
                 listing = ProductListing.objects.get(pk=listing_id)
-                listing.cart=None
+                listing.cart.clear()
                 listing.has_been_sold = True
                 listing.sold_date = datetime.now()
                 listing.save()
@@ -281,7 +281,7 @@ def get_queryset(self, *args, **kwargs):
 
         textbook = get_object_or_404(Textbook, isbn13=url_ibsn)
         product_listings = textbook.productlisting_set.all()
-        queryset = product_listings.filter(has_been_sold=False, cart=None)
+        queryset = product_listings.filter(has_been_sold=False)
 
         if url_ordering is not None:
             queryset = queryset.order_by(url_ordering)