encoder.encodeMap() should not encodeNull() when map length is zero

[wizardishungry]

Version of AWS SDK for Go?

2.0.0-preview.4

Version of Go (go version)?

go version go1.10.3 darwin/amd64

What issue did you see?

My Dynamo records are being created with NULL entries instead of empty maps.

Steps to reproduce

type Message struct {
	ID            string `json:"id"`
	Likes   map[int]string `json:"likes"`

}

// Create persist a message
func (d *MessageDao) Create(message *model.Message,) (*model.Message, error) {

	av, err := dynamodbattribute.MarshalMap(message)
	if err != nil {
		return nil, err
	}
	input := &dynamodb.PutItemInput{
		Item:      av,
		TableName: aws.String(d.tableName),
	}

	req := d.DynamoDB.PutItemRequest(input)
	_, err = req.Send()

	if err != nil {
		return nil, err
	}
	return message, nil
}

aws-sdk-go-v2/service/dynamodb/dynamodbattribute/encode.go

Lines 335 to 360 in ac26486

	func (e *Encoder) encodeMap(av *dynamodb.AttributeValue, v reflect.Value, fieldTag tag) error {
	av.M = map[string]dynamodb.AttributeValue{}
	for _, key := range v.MapKeys() {
	keyName := fmt.Sprint(key.Interface())
	if keyName == "" {
	return &InvalidMarshalError{msg: "map key cannot be empty"}
	}
	elemVal := v.MapIndex(key)
	elem := dynamodb.AttributeValue{}
	err := e.encode(&elem, elemVal, tag{})
	skip, err := keepOrOmitEmpty(fieldTag.OmitEmptyElem, elem, err)
	if err != nil {
	return err
	} else if skip {
	continue
	}
	av.M[keyName] = elem
	}
	if len(av.M) == 0 {
	encodeNull(av)
	}
	return nil
	}

[wizardishungry]

see aws/aws-sdk-go#682

[xibz]

Hello @wizardishungry, thank you for reaching out to us. We've received this same issue in v1, please see here. The workaround will also work for v2. I am going to go ahead and mark this as a feature request. If you have any more questions or issues with the workaround please let us know.

[wizardishungry]

For those arriving via search, etc. – a workaround for v2:

av, err := dynamodbattribute.MarshalMap(message)
if err != nil {
  return nil, err
}
emptyMap := make(map[string]dynamodb.AttributeValue)
av["some map"] = dynamodb.AttributeValue{M: emptyMap}

[jasdel]

This is related to the v1 SDK work, aws/aws-sdk-go#2419

[jasdel]

This feature was implemented in #401 and released in v0.13.0