Use reflection to get correct PK column name (#17)

Fixing [issue of using non-id primary key](#16) by [Marco Falcioni](https://github.com/marcofalcioni)

setup.py

@@ -13,7 +13,7 @@
 
 setup(
     name='django-pg-returning',
-    version='1.2.1',
+    version='1.2.2',
     packages=['django_pg_returning'],
     package_dir={'': 'src'},
     url='https://github.com/M1hacka/django-pg-returning',

src/django_pg_returning/manager.py

@@ -39,7 +39,7 @@ def _insert(self, objs, fields, return_id=False, raw=False, using=None, ignore_c
         query.insert_values(fields, objs, raw=raw)
 
         self.model._insert_returning_cache = self._execute_sql(query, return_fields, using=using)
-        return self.model._insert_returning_cache.values_list('id', flat=True) if return_id else None
+        return self.model._insert_returning_cache.values_list(self.model._meta.pk.column, flat=True) if return_id else None
 
     _insert.alters_data = True
     _insert.queryset_only = False
@@ -169,9 +169,9 @@ def bulk_create_returning(self, objs, batch_size=None):
         # Replace values fetched from returned data
         if result and result[0].pk:
             # For django 1.10+ where objects can be matched
-            values_dict = {item['id']: item for item in self.model._insert_returning_cache.values()}
+            values_dict = {item[self.model._meta.pk.column]: item for item in self.model._insert_returning_cache.values()}
             for item in result:
-                for k, v in values_dict[item.id].items():
+                for k, v in values_dict[item.pk].items():
                     setattr(item, k, v)
         else:
             # For django before 1.10 which doesn't fetch primary key