How to add item to a dynamodb list with golang

add-item-to-list-dynamodb.go

/*
Took me a while to figure this out, this should never be so difficult IMHO. The dynamodb api could use more examples in its doc.

The documentation for doing this is scattered in a few places:

1. dynamodb api doc for golang: https://docs.aws.amazon.com/sdk-for-go/api/service/dynamodb/
2. the Update Expression: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html

The structs belowed are abbreviated for the sake of the demo: adding a string to the questions slice in the dynamodb table.

Note you will HAVE to use if_not_exists if using list_append(), and you can only use it with SET, otherwise it would return this error:
"the document path provided in the update expression is invalid for update", 
apparently it won't work if the attribute is empty or null initally.
*/

type Test struct {
	Id            int       `json:"id"`
	Name          string    `json:"name"`
	Questions     []string  `dynamodbav:"questions,omitempty"`
}

func (dq *DynamodbQuestion) Add2Test(testId string) bool {
	sess, err := session.NewSession(&aws.Config{Region: aws.String(config.AWS_REGION), Endpoint: aws.String(config.AWS_DYNAMODB_ENDPOINT)})
	svc := dynamodb.New(sess)
	if err != nil {
		fmt.Println(err)
		return false
	}
	av := &dynamodb.AttributeValue{
		S: aws.String(dq.Id),
	}
	var qids []*dynamodb.AttributeValue
	qids = append(qids, av)

	input := &dynamodb.UpdateItemInput{
		Key: map[string]*dynamodb.AttributeValue{
			"id": {
				N: aws.String(testId),
			},
			"uid": {
				S: aws.String(dq.UserId),
			},
		},
		ExpressionAttributeValues: map[string]*dynamodb.AttributeValue{
			":qid": {
				L: qids,
			},
			":empty_list": {
				L: []*dynamodb.AttributeValue{},
			},
		},
		ReturnValues:     aws.String("ALL_NEW"),
		UpdateExpression: aws.String("SET questions = list_append(if_not_exists(questions, :empty_list), :qid)"),
		TableName:        aws.String("tests"),
	}

@Shuo-Li lol completely unrelated but i used to work with Alex DeBrie at Hudl, great dude, what a pleasant surprise, thanks for the trip down memory lane..

@wliao008 I've learnt a great deal from Alex's book, which is probably THE best book about the single-table design with dynamodb database

@jonny-rimek The document link you posted was the one that helped me figure out DELETE should have been used. Wish I could have read it earlier.