java DynamoDBMapper 仅在对象不存在时才保存
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DynamoDBMapper save only if object doesn't exist
提问by Nosrettap
Using the Java DynamoDBMapper, how can I save an object only if it doesn't already exist (based on primary key). If it does exist, I want an exception or failure to be thrown, rather than having the existing entry updated.
使用 Java DynamoDBMapper，如何仅在对象不存在（基于主键）时才保存该对象。如果确实存在，我希望抛出异常或失败，而不是更新现有条目。
回答by tddmonkey
I believe you should be able to do this with a DynamoDbSaveExpressionobject that can apply to the mapper.
我相信您应该能够使用可以应用于映射器的DynamoDbSaveExpression对象来做到这一点。
There's a tutorial on the AWS site here, code shown below:
AWS 站点上有一个教程here，代码如下所示：
try {
DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
Map expected = new HashMap();
expected.put("status", new ExpectedAttributeValue().withExists(false));
saveExpression.setExpected(expected);
mapper.save(obj, saveExpression);
} catch (ConditionalCheckFailedException e) {
// This means our save wasn't recorded, since our constraint wasn't met
// If this happens, the worker can simply look for a new task to work on
}
回答by anon58192932
Here's the correct way to implement this with the DynamoDBMapper:
以下是使用 DynamoDBMapper 实现此功能的正确方法：
User newUser = new User();
newUser.setUsername(username);
newUser.setPassword(password);
DynamoDBSaveExpression saveExpr = new DynamoDBSaveExpression();
saveExpr.setExpected(new ImmutableMap.Builder()
.put("username", new ExpectedAttributeValue(false)).build());
dynamoDBMapper.save(newUser, saveExpr);
Source: https://blog.jayway.com/2013/08/24/create-entity-if-not-exists-in-dynamodb-from-java/
来源：https: //blog.jayway.com/2013/08/24/create-entity-if-not-exists-in-dynamodb-from-java/
EDIT: Here's my implementation of it in practice. Extremely easy to implement:
编辑：这是我在实践中的实现。非常容易实施：
public Statement saveIfNotExist(Statement statement) throws ConditionalCheckFailedException {
return mapper.save(statement, new DynamoDBSaveExpression().withExpected(ImmutableMap.of("id", new ExpectedAttributeValue(false))));
}
with passing unit test:
通过单元测试：
@Test(expectedExceptions = ConditionalCheckFailedException.class)
public void shouldNotProcessIdenticalStatementUpdateInstances() {
Statement statement1 = StatementTestBuilder.valid().build();
Statement statement2 = StatementTestBuilder.valid().withId(statement1.getId()).build();
statementRepository.saveIfNotExist(statement1);
statementRepository.saveIfNotExist(statement2);
}
