Is there a way to enforce unique constraint on a property (field) other than the primary key in dynamodb

Question

In dynamodb, if you want to enforce uniqueness in a field other than the primary key (like were you have a users table and want unique email addresses for users while primary key is a userid which is a number) is there a way other thans scanning the table to see if the email is already in use?

Answer 1

Short answer: No.

DynamoDB is a key:value store. It is very good at quickly retrieving/saving Items because it does a couple of compromise. This is a constraint you have to handle yourself.

Nonethess, depending on your actual model, it might be a good idea to use this field as you hash_key or consider using a range_key

If this is not possible, I advise you to de-normalize your data. You currently have something like:

UserTable

• hash_key: user_id
• e-mail
• ...

To ensure unicity, add a new table with this schema:

EmailUser

• hash_key: e-mail
• user_id

To make sure an e-mail is unique, just issue a GetItem to EmailUser before.

This kind of de-normalization is quite common with No-SQL databases.

Answer 2

The DynamoDB per se does not support unique constraints, but you can somehow ensure uniqueness by using atomic counter and incorporate this counter value into your data.

In my case I have to make sure both username and userId do not have duplicates. username is my partition key so I won't have any problems using the attribute_not_exists(username) to prevent overwrites;

For userId I will first retrieve a new value from the atomic counter and then put it as my userId value. It may not be completely sequential but it can guarantee uniqueness in this sense.