How to update only a single field in DynamoDB using the AWS Go SDK

Question content

Hopefully someone can shed some light on this issue I've been struggling with.

I have this table in dynamo, let's call it people, in this table, I have the attribute of id as our partition key, name, lastname and status . I want to be able to update just a single property or save all properties of the id. Now, this is how I do it. I created the following structure:

type personupdate struct {
    firstname     string `json:"firstname,omitempty"`
    lastname      string `json:"lastname,omitempty"`
    status        string `json:"status,omitempty"`
}

The request from the server simply updates the person's last name, so our request body will look like this:

{
    "lastname": "bob"
}

After binding the request to the structure, the object that will be sent to dynamo will look like this:

{
    "firstname": "",
    "lastname": "bob",
    "status": "",
}

Now, as you can see, when it is time to write to dynamo, only one property should be updated and the rest of the null/null properties should be ignored.

The code written to do this can be condensed into the following:

// marshal our object
    _, err := dynamodbattribute.marshalmap(person)
    if err != nil{
        fmt.println("some error marshaling")
    }
    // create our update input
    input := &dynamodb.updateiteminput{
        key: map[string]*dynamodb.attributevalue{
            "id":{
                s: aws.string("1234"),
            },
        },
        tablename: aws.string("people"),
        expressionattributevalues: map[string]*dynamodb.attributevalue{
            ":fn": {
                s: aws.string(person.firstname),
            },
            ":ln":{
                s: aws.string(person.lastname),
            },
            ":st":{
                s: aws.string(person.status),
            },
        },
        updateexpression: aws.string("set firstname = :fn, lastname = :ln, status = :st"),
        returnvalues: aws.string("updated_new"),
    }
    // send update request to dynamo
    _, err := service.updateitem(input)

Now, the update works without any problem, the problem is that null values ​​for firstname and status are also passed. I've tried reading their documentation, but I'm still a little confused. I know the java sdk allows you to pass a flag called update_skip_null_attributes which allows you to skip those null values ​​and update only those values ​​that have data. I don't know what the equivalent is in go, any help/guidance would be great.

renew:

Try using the following model:

type usermodel struct {
    firstname string `json:"firstname,omitempty" dynamodbav:"firstname,omitempty"`
    lastname  string `json:"lastname,omitempty" dynamodbav:"lastname,omitempty"`
}

Follow the advice given by @jarmod and @fedonev. Logically and using the documentation it's understandable why this should work, unfortunately it doesn't

Decided to switch the sdk from v1 to v2 to see if updating it would help, although I'm stuck in the same predicament again. This is what my update function looks like.

update :=expression.Set(expression.Name("firstName"),expression.Value(user.FirstName))
    update.Set(expression.Name("lastName"), expression.Value(user.LastName))

    expr, err := expression.NewBuilder().WithUpdate(update).Build()

    if err != nil {
        "log error here..."
    }

    _, err = svc.UpdateItem(context.TODO(), &dynamodb.UpdateItemInput{
        TableName:                 aws.String("people"),
        Key:                       map[string]types.AttributeValue{"id": id},
        ExpressionAttributeNames:  expr.Names(),
        ExpressionAttributeValues: expr.Values(),
        UpdateExpression:          expr.Update(),
        ReturnValues:              types.ReturnValueUpdatedNew,
    })

    if err != nil {
        "log error here..."

    }

I ended up solving my problem by writing a query and create function that pretty much queries the json payload by the id we want to update and updates based on the difference of what we want the query to do, any empty fields will Replaced by query results. So far it solved my problem, but I'm still wondering how to update it with its intended functionality.

Correct answer

As @jarmod's comment says, when applying the dynamodbav:",omitempty" tag to the marshalling structure Skip zero value fields :

type personupdate struct {
    firstname     string `json:"firstname,omitempty" dynamodbav:",omitempty"`
    lastname      string `json:"lastname,omitempty" dynamodbav:",omitempty"`
    status        string `json:"status,omitempty" dynamodbav:",omitempty"`
}

[Edit: Add usage] marshalmap Zero value fields will now be ignored, respecting labels. Iterate over the map to construct an update expression:

av, _ := attributevalue.MarshalMap(person)

update := expression.UpdateBuilder{}

for k, v := range av {
        update = update.Set(expression.Name(k), expression.Value(v))
}

Build the expression and pass its output to updateitem as shown in the op.

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