如果值不存在或检查 DynamoDBMapper 中的条件，则更新

Question

I am working on a method that saves to DynamoDB. I want the method to save if the value doesn't exist in the table. If it does exist, I want to apply a conditional update. I am using DynamoDBMapper's save method.

The code I have at the moment does the conditional save successfully, but throws an exception if the column doesn't exist in the database.

Is there a way to come up with a conditional update expression that checks if the value doesn't exist or checks for the condition I need?

The code I have at the moment, which is in Java, looks like this:

DynamoDBSaveExpression saveExpression = new DynamoDBSaveExpression();
AttributeValue attributeValue = new AttributeValue(valueToSave);
ExpectedAttributeValue expectedAttributeValue = new ExpectedAttributeValue(attributeValue).withComparisonOperator(ComparisonOperator.LT);
Map<String, ExpectedAttributeValue> expected = new HashMap<>();
expected.put("key", expectedAttributeValue);
saveExpression.setExpected(expected);
dynamoDbMapper.save(objectToSave);

Thanks!

Answer 1

The ExpectedAttribute stuff is there for Legacy applications ( https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_PutItem.html#DDB-PutItem-request-Expected ).

It's recommended that you use the newer ConditionExpression instead.

This is one way i've done it, although the snippet below is converted from Scala code and untested.

Map attrVals = new HashMap() {{
  put(":revision", new AttributeValue().withN(revision.toString)));   
}};

PutItemRequest request = new PutItemRequest().withTableName(myTableName)
.withItem(myItem)
.withConditionExpression(attribute_not_exists(primaryKey) OR revision <= :revision)
.withExpressionAttributeValues(attrVals);

dynamoClient.putItem(request);