DynamoDBMapper save only if object doesn't exist

Question

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.

Solution 1

I believe you should be able to do this with a DynamoDbSaveExpression object that can apply to the mapper.

There's a tutorial on the AWS site here, code shown below:

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
}

Solution 2

Here's the correct way to implement this with the DynamoDBMapper:

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/

EDIT: Here's my implementation of it in practice. Extremely easy to implement:

public Statement saveIfNotExist(Statement statement) throws ConditionalCheckFailedException {
    return mapper.save(statement, new DynamoDBSaveExpression().withExpected(ImmutableMap.of("id", new ExpectedAttributeValue(false))));
}

with passing unit test:

@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);
}

Comments

[aalosious]

shouldn't it be withExists(false)?

[Sebastian D'Agostino]

A parenthesis is missing after ("READY") Here is the correct line: expected.put("status", new ExpectedAttributeValue(new AttributeValue("READY")).withExists(true));

[Corin]

-1 as this answer only saves when the item already exists in the table. This is the opposite of the result desired by the question.

[tddmonkey]

changed to fix the boolean state (oops)

[juan.facorro]

Edited the answer since when using .withExists(false) there must not be a value provided.

[ufdeveloper]

Does this make an additional read on the table? I believe it does!

[tddmonkey]

No - it's all handled during the write