问题描述

我正在使用 DynamoDBMapper，并且希望有条件地保存当且仅当 hashkey 和 range 键组合不存在时.我知道有一些方法可以使用 UUID 来减少发生冲突的可能性，但我想通过使用条件保存来保护自己.

I'm using DynamoDBMapper and would like to conditionally save if and only if the hashkey and range key combination does not exist. I know there are ways to use UUIDs to reduce the possibility of a collision but I would like to protect myself by using conditional saves.

我遇到了这篇文章 使用 DynamoDBSaveExpression 但是我无法指定条件是hashkey AND rangekey"不能存在.API 指定withConditionalOperator 方法，但我在课堂上看不到这个.我也在使用来自 here 的最新 aws java sdk.

I came across this article that uses DynamoDBSaveExpression however I'm not able to specify that the condition is "hashkey AND rangekey" cannot exist. The API specifies a withConditionalOperator method but I'm not able to see this in my class. I am using the latest aws java sdk also from here.

关于如何有条件地保存的任何建议?或者我可能做错了什么?

Any suggestions on how to conditionally save? Or what I may be doing incorrectly?

推荐答案

DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
Map<String, ExpectedAttributeValue> expectedAttributes = 
    ImmutableMap.<String, ExpectedAttributeValue>builder()
        .put("hashKey", new ExpectedAttributeValue(false))
        .put("rangeKey", new ExpectedAttributeValue(false))
        .build();
saveExpression.setExpected(expectedAttributes);
saveExpression.setConditionalOperator(ConditionalOperator.AND);
try {
    dynamoDBMapper.save(objectToSave, saveExpression);
} catch (ConditionalCheckFailedException e) {
    //Handle conditional check
}

这使用 public ExpectedAttributeValue(Boolean exists) 构造函数，它只是在内部调用 setExists.

This uses the public ExpectedAttributeValue(Boolean exists) constructor, which just internally calls setExists.

这篇关于用于条件保存的 DynamoDBMapper的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！