Allow VersionAttribute to operate on items with a version of zero (#869)

Author: bobwanglyft

pynamodb/models.py

@@ -936,7 +936,7 @@ def _handle_version_attribute(self, serialized_attributes, actions=None):
         version_attribute = self.get_attributes()[self._version_attribute_name]
         version_attribute_value = getattr(self, self._version_attribute_name)
 
-        if version_attribute_value:
+        if version_attribute_value is not None:
             version_condition = version_attribute == version_attribute_value
             if actions:
                 actions.append(version_attribute.add(1))

tests/test_model.py

@@ -3076,6 +3076,60 @@ def test_model_version_attribute_save(self):
 
             deep_eq(args, params, _assert=True)
 
+    def test_model_version_attribute_save_with_initial_version_zero(self):
+        self.init_table_meta(VersionedModel, VERSIONED_TABLE_DATA)
+        item = VersionedModel('test_user_name', email='test_user@email.com', version=0)
+
+        with patch(PATCH_METHOD) as req:
+            req.return_value = {}
+            item.save()
+            args = req.call_args[0][1]
+            params = {
+                'Item': {
+                    'name': {
+                        'S': 'test_user_name'
+                    },
+                    'email': {
+                        'S': 'test_user@email.com'
+                    },
+                    'version': {
+                        'N': '1'
+                    },
+                },
+                'ReturnConsumedCapacity': 'TOTAL',
+                'TableName': 'VersionedModel',
+                'ConditionExpression': '#0 = :0',
+                'ExpressionAttributeNames': {'#0': 'version'},
+                'ExpressionAttributeValues': {':0': {'N': '0'}}
+            }
+
+            deep_eq(args, params, _assert=True)
+            item.version = 1
+            item.name = "test_new_username"
+            item.save()
+            args = req.call_args[0][1]
+
+            params = {
+                'Item': {
+                    'name': {
+                        'S': 'test_new_username'
+                    },
+                    'email': {
+                        'S': 'test_user@email.com'
+                    },
+                    'version': {
+                        'N': '2'
+                    },
+                },
+                'ReturnConsumedCapacity': 'TOTAL',
+                'TableName': 'VersionedModel',
+                'ConditionExpression': '#0 = :0',
+                'ExpressionAttributeNames': {'#0': 'version'},
+                'ExpressionAttributeValues': {':0': {'N': '1'}}
+            }
+
+            deep_eq(args, params, _assert=True)
+
     def test_version_attribute_increments_on_update(self):
         self.init_table_meta(VersionedModel, VERSIONED_TABLE_DATA)
         item = VersionedModel('test_user_name', email='test_user@email.com')

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,pypy,pypy3
+envlist = py36,py37,py38,pypy,pypy3
 
 [testenv]
 deps = -rrequirements-dev.txt