如何使用祖先未知的键查询实体？

Question

A is ancestor of B

type A struct {}
type B struct {}

How would I search for B whose string ID is stringID, regardless of ancestor? (B and ancestor A may not even exist)

ctx := appengine.NewContext(r)
a := ??? // 'A' may not exist, so what do I do here?
k := datastore.NewKey(ctx, "B", "stringID", 0, a)
e := new(B)
if err := datastore.Get(ctx, k, e); err != nil {
    http.Error(w, err.Error(), 500)
    return
}

Answer 1

Ancestor entity may not exist, but its ID or Name must be known. Otherwise you will not be able to create a unique key for a child entity. Note that child of A and child of B may have the same ID or Name.

Also note that you may delete one entity and create a new one, but you cannot change the parent of an entity once it's created.