DynamoDB:如果putItem中不存在键，我该如何做putItem？

Question

我现在使用的是亚马逊AWS DynamoDB。在未来，我想把项目放在我的表中，但是仅仅是如果一个键相同的项目不存在，这样我就不会覆盖现有的值。你知道我怎么做到的吗？我的代码：

from __future__ import print_function # Python 2/3 compatibility
import boto3
import json
import decimal

# Helper class to convert a DynamoDB item to JSON.
class DecimalEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, decimal.Decimal):
            if o % 1 > 0:
                return float(o)
            else:
                return int(o)
        return super(DecimalEncoder, self).default(o)

dynamodb = boto3.resource('dynamodb', region_name='eu-central-1')

table = dynamodb.Table('Movies')

title = "The Big New Movie"
year = 2015

response = table.put_item(
    Item={
        'year': year,
        'title': title,
        'info': {
             'plot':"Nothing happens at all.",
             'rating': decimal.Decimal(0)
        }
    },
 )

我听说过ConditionExpression。但我不知道怎么把这个加上去。不是这样的：

response = table.put_item(
    Item={
        'year': year,
        'title': title,
        'info': {
             'plot':"Nothing happens at all.",
             'rating': decimal.Decimal(0)
        }
    },
     ConditionExpression = "attribute_not_exists",
 )

因为这样我就会得到以下错误：

Traceback (most recent call last):
  File "/Users/iTom/ownCloud/Documents/Workspace/PyCharm/DynamoDBTest/MoviesItemOps1.py", line 32, in <module>
ConditionExpression = "attribute_not_exists",
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/boto3/resources/factory.py", line 518, in do_action
response = action(self, *args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/boto3/resources/action.py", line 83, in __call__
response = getattr(parent.meta.client, operation_name)(**params)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/botocore/client.py", line 252, in _api_call
return self._make_api_call(operation_name, kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/botocore/client.py", line 542, in _make_api_call
raise ClientError(parsed_response, operation_name)
botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the PutItem operation: Invalid ConditionExpression: Syntax error; token: "<EOF>", near: "attribute_not_exists"

Answer 1

必须在条件表达式中指定一个属性，如下所示：

ConditionExpression = "attribute_not_exists(title)"