.update() will create new items

[jkav77]

This was brought up in #39, but is separate from the main issue there which is schema enforcement. Basically, you can create a new item in DynamoDB with an update call. The expected behavior should be to fail and only allow creating new items with a call to .create().

[deedw]

See comments against pull request #50. I believe existing functionality is as expected for DynamoDb and changes to this would break existing code.

[jkav77]

@deedw Given the underlying behavior of DynamoDB.updateItem(), I agree that the current behavior is as expected. In my projects, attempting to update a non-existent item has always represented an error case I would want to know about. This issue made sense to me in that context.

Having looked through the code now, it appears that you can do this easily with the param object like { expected: { id: { Exists: true } } }. I don't see this in the documentation, so I will change the PR to add this to the docs after I finish testing to make sure this actually works as I think it does.

[jkav77]

I updated the documentation in #53. This provides an easy way to modify the behavior of .update() to fail for items that don't exist. In fact, this was an undocumented feature already in the code. I added two tests to check the behavior works and described the parameter in the README.md.