type User struct {
    Id string
    Name string
    Pass string
    Email string
}

user := User {
    Id:     "4be0f045-6ce8-4d3e-751e-15bd84f2b925",
    Name:   "test",
    Pass:   "test",
    Email:  "",
}

datastore.Put(context, datastore.NewIncompleteKey(context, "User", nil), &user)

user.Email = "new@email.com"
filter := string.Format("Id='{0}'", "4be0f045-6ce8-4d3e-751e-15bd84f2b925");
datastore.Put(context, "User", user, filter ), &user)

querySize := 1000
query := datastore.NewQuery("User").Limit(querySize)
queryResult := make([]User, 0, querySize)
query.GetAll(context, &queryResult)

query := datastore.NewQuery("User").Filter("ID =", "4be0f045-6ce8-4d3e-751e-15bd84f2b925")
var u []User
key, err := query.GetAll(context, &u)

u[0].Email = "new.email@org"
datastore.Put(context,key[0],&u[0])

datastore.Delete(context,key[0])