Add utility for unmarshalling dynamodb Query Items.

[sanathp]

Query returns items in the below format

Items []map[string]*AttributeValue

I cannot see any UnMarshal function which supports the datatype []map[string]*AttributeValue

Should I UnMarshal each element in a for loop ?? whats the best way to do this?

[jasdel]

Hi @sanathp thanks for contacting us. You are correct the SDK does not currently provide an utility method to unmarshal a slice of map[string]*dynamodb.AttributeValue. The best way to unmarshal this is to iterate through the returned list calling dynamodbattributevalue.UnmarshalMap() for each element in the slice.

[jasdel]

Marking this as a Feature Request to make receiving items from query API operations easier to unmarshal into Go value types. We're also always glad to review PR if you're looking to contribute this feature to the SDK.

[twoism]

I ran into this same issue. It seems strange to not have a native way to unmarshal the return types of both QueryOutput.Items and ScanOutput.Items. This was my solution for now but it would be nice to not have to iterate over the slice in both this code and UnmarshalList.

func UnmarshalListOfMaps(l []map[string]*dynamodb.AttributeValue, out interface{}) error {
    attrs := make([]*dynamodb.AttributeValue, len(l))

    for i, m := range l {
        attrs[i] = &dynamodb.AttributeValue{M: m}
    }

    return dynamodbattribute.UnmarshalList(attrs, out)
}

[twoism]

Not to harp on this but asking end users to incur N*2 marshaling time for a common return type doesn't seem like the answer. I did some digging and given that the above code is essentially just upcasting each item in []map[string]*dynamodb.AttributeValue into an AttributeValue, couldn't QueryOutput.Items just return a []*dynamodb.AttributeValue where each item has the value type of M? Unless I am wrong, these are essentially the same thing. If so, the marshaler would not need to be updated. It would break the API but I think using AWS types would make things easier. Just a thought and I could be missing something about why the map is necessary, happy to help work on this if needed :)

[jasdel]

Thanks for the feedback @twoism. I'll forward your feedback along to the DynamoDB team to help improve the API. We wouldn't be able to change the API due to the breaking change without the service doing a major version bump.

To resolve this feature request I think would solve it similar to the example you posted, same for encoding. The SDK already does this to a more limited degree for marshal of list and maps. So updating to unmarshal a list of maps is pretty straightforward, but like you mentioned has more complexity.

I'm not sure yet how the marshaler side of this would work yet though. Not sure its even needed though, but may make sense for parity.

[jasdel]

Thanks for bringing up this issue. Change #897 Fixes this issue by adding a UnmarshalListOfMaps utility to the dynamodbattribute package. This will make using Scan and Query easier since you'll be able to unmarshal Items now without extra logic.

[twoism]

This is awesome, thanks a ton!

[timogoosen]

I know this is an old issue but could someone explain the usage ?