Go has-many relationship in Google App Engine Datastore

I am very new to Go and so far I am loving it. However, I can't figure this out for the life of me.

I have a simple package, invoice.

type Invoice struct {
    key datastore.Key
    Name string
    Created time.Time
    Updated time.Time
    lineItems []LineItem
}

An Invoice has several LineItems.

type LineItem struct {
	key datastore.Key
    InvoiceKey *datastore.Key
    Name string
    Description string
}

My package has a couple functions.

func New(c appengine.Context) (i Invoice)
func (i *Invoice) Update()
func (i *Invoice) Save(c appengine.Context)
func (i *Invoice) AddLineItem(c appengine.Context, name string)

New() returns a new Invoice, and saves the key generated by datastore.NewIncompleteKey() inside the Invoice as an unexported variable so that I can save it later.

All of that runs fine, whether it is the proper way to do this or not is another question. I'm open to comments on that too. What I can't seem to figure out is that last function.

func (i *Invoice) AddLineItem(c appengine.Context, name string) {
	key := datastore.NewIncompleteKey(c, "LineItem", &i.key)
	lineItem := LineItem{key: *key, InvoiceKey: &i.key, Name: name}
	i.lineItems = append(i.lineItems, lineItem)
}

And in Save()

for _, lineItem := range i.lineItems {
	_, err := datastore.Put(c, &lineItem.key, &lineItem)
	if err != nil {
		panic(err)
	}
}

I keep getting an invalid key error here. I'm basically just trying to make an Invoice have the ability to have many LineItems. Be able to save them all to the datastore, and then pull out the entire Invoice with all the LineItems as needed.

Am I on the right track? Is there a better way to do this?

Your lineItems []LineItem and key datastore.Key are unexported, they need to start with a capital letter otherwise the GAE won't be able to use it.

From the specs:
> An identifier may be exported to permit access to it from another package. An identifier is exported if both:
>
> 1. the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
> 2. the identifier is declared in the package block or it is a field name or method name.
>
> All other identifiers are not exported.

Don't use datastore.Key, use *datastore.Key instead. Any time you think it might not be set is a source of a bug, and will probably turn your invalid key errors into panics.

You are giving LineItem keys a parent (the Invoice), which is fine, but why then do you store the Invoice key in addition to the LineItem key? You can get the former by calling the latter's Parent method.

The issue I was having was trying to use an incomplete invoice key to generate an incomplete LineItem key.

key, err := datastore.Put(c, &i.key, i)

Use the complete key returned from the put.

for _, lineItem := range i.lineItems {
	key := datastore.NewIncompleteKey(c, "LineItem", key)
	lineItem.InvoiceKey = k
	_, err := datastore.Put(c, key, &lineItem)
	if err != nil {
		panic(err)
	}
}

That stores them. Now to the task of figuring out how to read them all out of the datastore! Wish me luck.