added support for specifying the ReturnValues option in update

[manast]

Model.update always returns all the new values in every call. This can have a severe performance penalty in some scenarios. This PR provides a backwards compatible option returnValues when calling update so that the 5 different settings provided by Dynamo can be used:

• NONE - If ReturnValues is not specified, or if its value is NONE, then nothing is returned. (This setting is the default for ReturnValues.)
• ALL_OLD - Returns all of the attributes of the item, as they appeared before the UpdateItem operation.
• UPDATED_OLD - Returns only the updated attributes, as they appeared before the UpdateItem operation.
• ALL_NEW - Returns all of the attributes of the item, as they appear after the UpdateItem operation.
• UPDATED_NEW - Returns only the updated attributes, as they appear after the UpdateItem operation.

[coveralls]

Coverage increased (+0.1%) to 82.61% when pulling c9d2d87 on manast:feature/support-for-specifying-return-values into 388662c on automategreen:master.

[fishcharlie]

@manast This looks amazing. Thanks so much!! Is there any chance you could add documentation regarding this new feature? We should also probably confirm to make sure there is a test to ensure backwards compatibility, and if not we should add one to confirm the backwards compatibility.

We will try to get this in for version 0.9. Thanks again!!

[fishcharlie]

LGTM

[fishcharlie]

Thanks again @manast! We'll get this in for version 0.9.

[dobrynin]

@manast, @fishcharlie

Can we add a note to the documentation clearly stating that the Dynamoose default differs from the DynamoDB default? The current phrasing led me to believe that NONE was the default for Dynamoose.

Also, what do you think about adding an option defaultReturnValues so that you can globally override the current default of ALL_NEW?

[fishcharlie]

@dobrynin Both ideas sound great! PR would be awesome.