DynamoDB - Put item if hash (or hash and range combination) doesn't exist

Question

Here are my use cases: I have a Dynamo table with a hash + range key. When I put new items in the table, I want to do a uniqueness check. Sometimes I want to guarantee th

Answer 1

Following Jun711's answer, here is what I did to implement putItem only if hash doesn't exist in Kotlin. DocId is the hash key of my DynamoDB table.

val item = Item().withPrimaryKey(...).withString(...)
val putItemSpec = PutItemSpec().withItem(item)
                    .withConditionExpression("attribute_not_exists(DocId)")
table.putItem(putItemSpec)

If using DynamoDBMapper annotations, here is an example.

@DynamoDBTable(tableName = "Docs")
class Docs {
    // Partition key
    @get:DynamoDBHashKey(attributeName = "DocId")
    var docId: String? = null
}

val doc = Docs().apply {
    docId = "myId"
}

val mapper = DynamoDBMapper(AmazonDynamoDBClientBuilder.defaultClient())

// As suggested by http://rrevo.github.io/2018/03/09/dynamo-no-update/
val ifDocIdNotExists = DynamoDBSaveExpression().apply {
    expected = mapOf("DocId" to ExpectedAttributeValue().apply {
        isExists = false
    })
}

mapper.save(doc, ifDocIdNotExists)

Catch com.amazonaws.services.dynamodbv2.model.ConditionalCheckFailedException to handle the case where the hash key already exists.