From 2609f6cd3a113292e2022deaf176e8656dff93a3 Mon Sep 17 00:00:00 2001
From: Terry Cain <terrycain@users.noreply.github.com>
Date: Fri, 17 Nov 2017 08:49:59 +0000
Subject: [PATCH] Added support for dynamo update_item on nested dicts (#1345)

---
 moto/dynamodb2/models.py              | 33 ++++++++++++++-
 tests/test_dynamodb2/test_dynamodb.py | 61 +++++++++++++++++++++++++++
 2 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/moto/dynamodb2/models.py b/moto/dynamodb2/models.py
index a4d8feb3c..0a48c277a 100644
--- a/moto/dynamodb2/models.py
+++ b/moto/dynamodb2/models.py
@@ -149,9 +149,38 @@ class Item(BaseModel):
                     key = key.strip()
                     value = value.strip()
                     if value in expression_attribute_values:
-                        self.attrs[key] = DynamoType(expression_attribute_values[value])
+                        value = DynamoType(expression_attribute_values[value])
                     else:
-                        self.attrs[key] = DynamoType({"S": value})
+                        value = DynamoType({"S": value})
+
+                    if '.' not in key:
+                        self.attrs[key] = value
+                    else:
+                        # Handle nested dict updates
+                        key_parts = key.split('.')
+                        attr = key_parts.pop(0)
+                        if attr not in self.attrs:
+                            raise ValueError()
+
+                        last_val = self.attrs[attr].value
+                        for key_part in key_parts:
+                            # Hack but it'll do, traverses into a dict
+                            if list(last_val.keys())[0] == 'M':
+                                last_val = last_val['M']
+
+                            if key_part not in last_val:
+                                raise ValueError()
+
+                            last_val = last_val[key_part]
+
+                        # We have reference to a nested object but we cant just assign to it
+                        current_type = list(last_val.keys())[0]
+                        if current_type == value.type:
+                            last_val[current_type] = value.value
+                        else:
+                            last_val[value.type] = value.value
+                            del last_val[current_type]
+
                 elif action == 'ADD':
                     key, value = value.split(" ", 1)
                     key = key.strip()
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index 17c5310d4..05daf23aa 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -1006,3 +1006,64 @@ def test_query_missing_expr_names():
 
     resp['Count'].should.equal(1)
     resp['Items'][0]['client']['S'].should.equal('test2')
+
+
+# https://github.com/spulec/moto/issues/1342
+@mock_dynamodb2
+def test_update_item_on_map():
+    dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
+
+    # Create the DynamoDB table.
+    dynamodb.create_table(
+        TableName='users',
+        KeySchema=[
+            {
+                'AttributeName': 'forum_name',
+                'KeyType': 'HASH'
+            },
+            {
+                'AttributeName': 'subject',
+                'KeyType': 'RANGE'
+            },
+        ],
+        AttributeDefinitions=[
+            {
+                'AttributeName': 'forum_name',
+                'AttributeType': 'S'
+            },
+            {
+                'AttributeName': 'subject',
+                'AttributeType': 'S'
+            },
+        ],
+        ProvisionedThroughput={
+            'ReadCapacityUnits': 5,
+            'WriteCapacityUnits': 5
+        }
+    )
+    table = dynamodb.Table('users')
+
+    table.put_item(Item={
+        'forum_name': 'the-key',
+        'subject': '123',
+        'body': {'nested': {'data': 'test'}},
+    })
+
+    resp = table.scan()
+    resp['Items'][0]['body'].should.equal({'nested': {'data': 'test'}})
+
+    table.update_item(Key={
+        'forum_name': 'the-key',
+        'subject': '123'
+        },
+        UpdateExpression='SET body.#nested.#data = :tb',
+        ExpressionAttributeNames={
+            '#nested': 'nested',
+            '#data': 'data'
+        },
+        ExpressionAttributeValues={
+            ':tb': 'new_value'
+    })
+
+    resp = table.scan()
+    resp['Items'][0]['body'].should.equal({'nested': {'data': 'new_value'}})