From bdd4ae824b9dcf954352c190c7a770208105e07b Mon Sep 17 00:00:00 2001
From: Paul Craciunoiu <paul@craciunoiu.net>
Date: Thu, 14 Jan 2016 15:44:28 -0700
Subject: [PATCH] Support ADD for numeric values

---
 moto/dynamodb2/models.py                      | 11 ++++
 .../test_dynamodb_table_with_range_key.py     | 54 +++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index 950b0cf1d..20e6f7b33 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 from collections import defaultdict
 import datetime
+import decimal
 import json
 
 from moto.compat import OrderedDict
@@ -142,6 +143,16 @@ class Item(object):
                         del self.attrs[attribute_name]
                 else:
                     self.attrs[attribute_name] = DynamoType({"S": new_value})
+            elif action == 'ADD':
+                if set(update_action['Value'].keys()) == set(['N']):
+                    existing = self.attrs.get(attribute_name, DynamoType({"N": '0'}))
+                    self.attrs[attribute_name] = DynamoType({"N": str(
+                            decimal.Decimal(existing.value) +
+                            decimal.Decimal(new_value)
+                    )})
+                else:
+                    # TODO: implement other data types
+                    raise NotImplementedError('ADD not supported for %s' % ', '.join(update_action['Value'].keys()))
 
 
 class Table(object):
diff --git a/tests/test_dynamodb2/test_dynamodb_table_with_range_key.py b/tests/test_dynamodb2/test_dynamodb_table_with_range_key.py
index 4346a85bf..6a5010bb5 100644
--- a/tests/test_dynamodb2/test_dynamodb_table_with_range_key.py
+++ b/tests/test_dynamodb2/test_dynamodb_table_with_range_key.py
@@ -888,6 +888,60 @@ def test_update_item_does_not_exist_is_created():
     })
 
 
+@mock_dynamodb2
+def test_update_item_add_value():
+    table = _create_table_with_range_key()
+
+    table.put_item(Item={
+        'forum_name': 'the-key',
+        'subject': '123',
+        'numeric_field': Decimal('-1'),
+    })
+
+    item_key = {'forum_name': 'the-key', 'subject': '123'}
+    table.update_item(
+        Key=item_key,
+        AttributeUpdates={
+            'numeric_field': {
+                'Action': u'ADD',
+                'Value': Decimal('2'),
+            },
+        },
+    )
+
+    returned_item = dict((k, str(v) if isinstance(v, Decimal) else v)
+                         for k, v in table.get_item(Key=item_key)['Item'].items())
+    dict(returned_item).should.equal({
+        'numeric_field': '1',
+        'forum_name': 'the-key',
+        'subject': '123',
+    })
+
+
+@mock_dynamodb2
+def test_update_item_add_value_does_not_exist_is_created():
+    table = _create_table_with_range_key()
+
+    item_key = {'forum_name': 'the-key', 'subject': '123'}
+    table.update_item(
+        Key=item_key,
+        AttributeUpdates={
+            'numeric_field': {
+                'Action': u'ADD',
+                'Value': Decimal('2'),
+            },
+        },
+    )
+
+    returned_item = dict((k, str(v) if isinstance(v, Decimal) else v)
+                         for k, v in table.get_item(Key=item_key)['Item'].items())
+    dict(returned_item).should.equal({
+        'numeric_field': '2',
+        'forum_name': 'the-key',
+        'subject': '123',
+    })
+
+
 @mock_dynamodb2
 def test_boto3_query_gsi_range_comparison():
     table = _create_table_with_range_key()