Feature Request: Allow conditional writes

[derekmok]

Hi Sebastian,

Awesome work on this project. I have been using it at my company with success and I have a feature request/some thoughts that I'd like to run by you.

The specific problem that I want to solve is to be able to perform conditional writes through the library https://aws.amazon.com/blogs/developer/specifying-conditional-constraints-with-amazon-dynamodb-mapper/.

I thought I could do this by using a custom repository implementation, which kind of works, but is not so nice.

Ideally, my custom repository implementation would require a DynamoDBTemplate to be injected, and use the template to perform the conditional write. However, DynamoDBTemplate, does not expose a save method that accepts a DynamoDBSaveExpression, which is the class that ultimately is needed by the DynamoDBMapper for one to specify a conditional write.

Furthermore, the DynamoDBMapper that is inside the DynamoDBTemplate is not exposed in the spring context, so I cannot inject an instance of DynamoDBMapper into my custom repository either.

So, in my application, I end up creating a separate DynamoDBMapper instance that is available in my spring context, inject that into my custom repository, and then the conditional write works.

I feel like the client shouldn't be required to instantiate their own DynamoDBMapper to achieve this. I think this library can either:

1. Expose the DynamoDBMapper inside DynamoDBTemplate in the spring context, or if the mapper was intentionally hidden as a design choice, then:
2. Expose a save method in DynamoDBTemplate that accepts a DynamoDBSaveExpression.

Would be great to hear your thoughts on this. I'm happy to help with this issue and submit a PR based on which solution you think is better, or happy consider any other approaches as well.

Thanks, Derek

[t-patt]

Any thoughts on this, @derjust ?

[PLT875]

I would also see the benefits.

public void save(Object entity, DynamoDBSaveExpression saveExpression) in DynamoDBTemplate

Any issues?