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 glad I could help, and you solved your problem. REMOVE is for LISTS, which can contain both numbers and strings, DELETE is for SETS, which only contain either numbers or strings (and binary sets too afaik)

Unfortunately, the docs aren't super clear on that https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.DELETE

@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.