Java dynamodb：如何过滤所有没有特定属性的项目？
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dynamodb: how to filer all items which do not have a certain attribute?
提问by Aliza
I have a table of users with a primary hash key of userId. each user may/may not have a string attribute called "environment". I would like to get all the users which have "environment"="xyz" or which do not have the "environment" attribute.
我有一个用户表，其主哈希键为 userId。每个用户可能/可能没有一个名为“环境”的字符串属性。我想获得所有具有“环境”=“xyz”或没有“环境”属性的用户。
The following code will filter those users with environment=xyz, but how do I filter those items with no environment at all? Dynamo API will not allow to filter on an empty String.
以下代码将使用 environment=xyz 过滤那些用户，但是如何过滤那些根本没有环境的项目？Dynamo API 不允许过滤空字符串。
AmazonDynamoDBClient client = DbClientManager.getDynamoDbClient();
ArrayList<AttributeValue> avList = new ArrayList<AttributeValue>();
avList.add(new AttributeValue().withS("xyz"));
Condition scanFilterCondition = new Condition()
.withComparisonOperator(ComparisonOperator.EQ.toString())
.withAttributeValueList(avList);
Map<String, Condition> conditions = new HashMap<>();
conditions.put("environment", scanFilterCondition);
ScanRequest scanRequest = new ScanRequest()
.withTableName("users")
.withAttributesToGet(
"userId",
"environment");
.withScanFilter(conditions);
ScanResult result = client.scan(scanRequest);
For now I just dropped the scan filter, and I do the filtering client-side. Bit is there any way to do it server side?
现在我只是放弃了扫描过滤器，我在客户端进行过滤。位有什么办法可以做到服务器端？
Thanks, Aliza
谢谢，艾丽莎
回答by readyornot
You need to use the NULLComparisonOperator.
您需要使用NULL比较运算符。
Check out this link: http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Condition.html
查看此链接：http: //docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Condition.html
- NOT_NULL : The attribute exists.
- NULL : The attribute does not exist.
- NOT_NULL ：属性存在。
- NULL ：该属性不存在。
Does this work for you?
这对你有用吗？
回答by Vadim
Hope I'm not too late. I've found useful function which you could use in the query. I did not check with ScanRequest but with QueryRequest works as charm.
希望我还不算太晚。我找到了可以在查询中使用的有用函数。我没有检查 ScanRequest，但 QueryRequest 很有魅力。
QueryRequest queryRequest = new QueryRequest()
.withTableName("YouTableName")
queryRequest.setFilterExpression(" attribute_not_exists(yourAttributeName) ")
queryRequest.setExpressionAttributeValues(expressionAttributeValues)
queryRequest.setExclusiveStartKey(ifYouHave)
queryRequest.setSelect('ALL_ATTRIBUTES')
queryRequest.setExpressionAttributeNames(youNames)
attribute_not_exists(yourAttributeName) works with ":aws-sdk:1.11.11" also you could use attribute_exists(yourAttributeName)
attribute_not_exists(yourAttributeName) 适用于 ":aws-sdk:1.11.11" 你也可以使用 attribute_exists(yourAttributeName)
回答by Sahil
I my case also, attribute_not_exists(attribute) worked. You can refer to this question:- How to check whether an attribute is not present in dynamoDB filter expression
我的情况也是如此，attribute_not_exists(attribute) 起作用了。您可以参考这个问题：-如何检查 dynamoDB 过滤器表达式中是否不存在属性
for more details.
更多细节。
