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 that the hash is unique (ignoring the range). Other times I want to allow duplicate hashes, but guarantee that the hash and range combination is unique. How can I accomplish this?

I experimented with attribute_not_exists. It seems to handle the second case, where it checks the hash + key combination. Here's a PHP sample:

$client->putItem(array(     'TableName' => 'test',     'Item' => array(         'hash' => array('S' => 'abcdefg'),         'range' => array('S' => 'some other value'),         'whatever' => array('N' => 233)     ),     'ConditionExpression' => 'attribute_not_exists(hash)' )); 

Oddly, it doesn't seem to matter if I use attribute_not_exists(hash) or attribute_not_exists(range). They both seem to do exactly the same thing. Is this how it's supposed to work?

Any idea how to handle the case where I only want to check hash for uniqueness?

Answer 1

You can't. All items in DynamoDB are indexed by either their hash or hash+range (depending on your table).

A sort of summary of what is going on so far:

• A single hash key can have multiple range keys.
• Every item has both a hash and a range key
• You are making a PutItem request and must provide both the hash and range
• You are providing a ConditionExpression with attribute_not_exists on either the hash or range attribute name
• The attribute_not_exists condition is merely checking if an attribute with that name exists, it doesn't care about the value

Let's walk through an example. Let's start with a hash+range key table with this data:

1. hash=A,range=1
2. hash=A,range=2

There are four possible cases:

1. If you try to put an item with hash=A,range=3 and attribute_not_exists(hash), the PutItem will succeed because attribute_not_exists(hash) evaluates to true. No item exists with key hash=A,range=3 that satisfies the condition of attribute_not_exists(hash).

2. If you try to put an item with hash=A,range=3 and attribute_not_exists(range), the PutItem will succeed because attribute_not_exists(range) evaluates to true. No item exists with key hash=A,range=3 that satisfies the condition of attribute_not_exists(range).

3. If you try to put an item with hash=A,range=1 and attribute_not_exists(hash), the PutItem will fail because attribute_not_exists(hash) evaluates to false. An item exists with key hash=A,range=1 that does not satisfy the condition of attribute_not_exists(hash).

4. If you try to put an item with hash=A,range=1 and attribute_not_exists(range), the PutItem will fail because attribute_not_exists(range) evaluates to false. An item exists with key hash=A,range=1 that does not satisfy the condition of attribute_not_exists(range).

This means that one of two things will happen:

1. The hash+range pair exists in the database.
   • attribute_not_exists(hash) must be true
   • attribute_not_exists(range) must be true
2. The hash+range pair does not exist in the database.
   • attribute_not_exists(hash) must be false
   • attribute_not_exists(range) must be false

In both cases, you get the same result regardless of whether you put it on the hash or the range key. The hash+range key identifies a single item in the entire table, and your condition is being evaluated on that item.

You are effectively performing a "put this item if an item with this hash+range key does not already exist".