Java DynamoDB——仅在键不存在时插入（无映射器）
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Java DynamoDB -- Only insert if key not already present (without mapper)
提问by Jeremy
I only want to insert this row if the key is not present. I don't want to override the row if the key already exists.
如果键不存在，我只想插入这一行。如果键已经存在，我不想覆盖该行。
My syntax is this:
我的语法是这样的：
new PutItemRequest().withTableName(myTableName).withItem(myItem).withConditionExpression(?)
As per the AWS docs, I'd use something like attribute ATTRIBUTE_NOT_EXISTS. I could also use ComparisonOperator.NULL, etc. That's as far as I can understand.
根据AWS docs，我会使用类似属性 ATTRIBUTE_NOT_EXISTS 的东西。我也可以使用ComparisonOperator.NULL 等。这是我所能理解的。
Syntax tips? Some explanation of this withConditionExpression mechanism?
语法提示？用ConditionExpression机制对此进行一些解释？
采纳答案by zapl
The doc says ConditionExpressionreplaces the legacy ConditionalOperator(.NULL). You can go both routes but you're supposed to use .withConditionExpression(...)rather then the operand route.
文档说ConditionExpression替换了旧版ConditionalOperator( .NULL)。你可以走两条路线，但你应该使用.withConditionExpression(...)而不是操作数路线。
There is also https://java.awsblog.com/post/TxBG87QOQZRZJF/DynamoDB-XSpec-APIfor more complex expressions.
还有用于更复杂表达式的https://java.awsblog.com/post/TxBG87QOQZRZJF/DynamoDB-XSpec-API。
But in your case it should work by writing
但在你的情况下，它应该通过写作来工作
.withConditionExpression("attribute_not_exists(thingId)")
assuming your hash key property is named thingId. That approach is also documented here: http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_PutItem.html
假设您的哈希键属性名为thingId. 该方法也记录在此处：http: //docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_PutItem.html
To prevent a new item from replacing an existing item, use a conditional expression that contains the attribute_not_exists function with the name of the attribute being used as the HASH key for the table. Since every record must contain that attribute, the attribute_not_exists function will only succeed if no matching item exists.
要防止新项目替换现有项目，请使用包含 attribute_not_exists 函数的条件表达式，该函数的名称用作表的 HASH 键。由于每条记录都必须包含该属性，因此 attribute_not_exists 函数仅在不存在匹配项时才会成功。
回答by Sindhu
We can define Dynamo Db primary Key as combination of (partition key and sort key) . based on this assumption ,this code would throw away duplicate records .
我们可以将 Dynamo Db 主键定义为 (partition key and sort key) 的组合。基于这个假设，这段代码会丢弃重复的记录。
PrimaryKey primaryKey = new PrimaryKey();
primaryKey.addComponent("Partition_Key", partitionId);
primaryKey.addComponent("Sort_Key",sortId);
Item item = new Item().withPrimaryKey(primaryKey).withString("username", userName).withLong("time", time);
try {
PutItemSpec putItemSpec = new PutItemSpec().withItem(item).withConditionExpression("attribute_not_exists(Sort_Key)");
table.putItem(putItemSpec);
} catch(ConditionalCheckFailedException ex) {
logger.error("Record already exists in Dynamo DB Table ");
}
