The memoization doesn't work when objects don't have a primary key.

apps/permission/backends.py

@@ -114,8 +114,16 @@ class PermissionBackend(ModelBackend):
             query = query | perm.query
         return query
 
+    @staticmethod
     @memoize
-    def has_perm(self, user_obj, perm, obj=None):
+    def check_perm(user_obj, perm, obj=None):
+        """
+        Check is the given user has the permission over a given object.
+        The result is then memoized.
+        Exception: for add permissions, since the object is not hashable since it doesn't have any
+        primary key, the result is not memoized. Moreover, the right could change
+        (e.g. for a transaction, the balance of the user could change)
+        """
         if user_obj is None or isinstance(user_obj, AnonymousUser):
             return False
 
@@ -134,10 +142,13 @@ class PermissionBackend(ModelBackend):
         perm_field = perm[2] if len(perm) == 3 else None
         ct = ContentType.objects.get_for_model(obj)
         if any(permission.applies(obj, perm_type, perm_field)
-               for permission in self.permissions(user_obj, ct, perm_type)):
+               for permission in PermissionBackend.permissions(user_obj, ct, perm_type)):
             return True
         return False
 
+    def has_perm(self, user_obj, perm, obj=None):
+        return PermissionBackend.check_perm(user_obj, perm, obj)
+
     def has_module_perms(self, user_obj, app_label):
         return False
 

apps/permission/decorators.py

@@ -49,7 +49,10 @@ def memoize(f):
             # lru_cache makes the job of memoization
             # We store only the 512 latest data per session. It has to be enough.
             sess_funs[sess_key] = lru_cache(512)(f)
-        return sess_funs[sess_key](*args, **kwargs)
+        try:
+            return sess_funs[sess_key](*args, **kwargs)
+        except TypeError:  # For add permissions, objects are not hashable (not yet created). Don't memoize this case.
+            return f(*args, **kwargs)
 
     func.func_name = f.__name__
 

apps/permission/permissions.py

@@ -44,7 +44,7 @@ class StrongDjangoObjectPermissions(DjangoObjectPermissions):
 
         perms = self.get_required_object_permissions(request.method, model_cls)
         # if not user.has_perms(perms, obj):
-        if not all(PermissionBackend().has_perm(user, perm, obj) for perm in perms):
+        if not all(PermissionBackend.check_perm(user, perm, obj) for perm in perms):
             # If the user does not have permissions we need to determine if
             # they have read permissions to see 403, or not, and simply see
             # a 404 response.

apps/permission/signals.py

@@ -44,7 +44,7 @@ def pre_save_object(sender, instance, **kwargs):
         # We check if the user can change the model
 
         # If the user has all right on a model, then OK
-        if PermissionBackend().has_perm(user, app_label + ".change_" + model_name, instance):
+        if PermissionBackend.check_perm(user, app_label + ".change_" + model_name, instance):
             return
 
         # In the other case, we check if he/she has the right to change one field
@@ -56,11 +56,11 @@ def pre_save_object(sender, instance, **kwargs):
             # If the field wasn't modified, no need to check the permissions
             if old_value == new_value:
                 continue
-            if not PermissionBackend().has_perm(user, app_label + ".change_" + model_name + "_" + field_name, instance):
+            if not PermissionBackend.check_perm(user, app_label + ".change_" + model_name + "_" + field_name, instance):
                 raise PermissionDenied
     else:
         # We check if the user has right to add the object
-        has_perm = PermissionBackend().has_perm(user, app_label + ".add_" + model_name, instance)
+        has_perm = PermissionBackend.check_perm(user, app_label + ".add_" + model_name, instance)
 
         if not has_perm:
             raise PermissionDenied
@@ -87,5 +87,5 @@ def pre_delete_object(instance, **kwargs):
     model_name = model_name_full[1]
 
     # We check if the user has rights to delete the object
-    if not PermissionBackend().has_perm(user, app_label + ".delete_" + model_name, instance):
+    if not PermissionBackend.check_perm(user, app_label + ".delete_" + model_name, instance):
         raise PermissionDenied

apps/permission/templatetags/perms.py

@@ -54,7 +54,7 @@ def model_list(model_name, t="view"):
 
 
 def has_perm(perm, obj):
-    return PermissionBackend().has_perm(get_current_authenticated_user(), perm, obj)
+    return PermissionBackend.check_perm(get_current_authenticated_user(), perm, obj)
 
 
 def can_create_transaction():
@@ -77,7 +77,7 @@ def can_create_transaction():
         amount=0,
         reason="Check permissions",
     )
-    session["can_create_transaction"] = PermissionBackend().has_perm(user, "note.add_transaction", empty_transaction)
+    session["can_create_transaction"] = PermissionBackend.check_perm(user, "note.add_transaction", empty_transaction)
     return session.get("can_create_transaction") == 1