This shows a failing test that I'm unable to figure out what I'm doing wrong.

gae_test.go

package hello

import (
	"appengine/aetest"
	"appengine/datastore"
	"reflect"
	"testing"
)

type Foo struct {
	Account string
}

func TestScheduleCraziness(t *testing.T) {
	collectionName := "Foo"
	accountName := "user@example.com"

	ctx, err := aetest.NewContext(nil)
	if err != nil {
		t.Fatalf("Failed to create context: %v", err)
	}
	defer ctx.Close()

	item := Foo{Account: "admin@gmail.com"}

	ancestorKey := datastore.NewKey(ctx, "Account", accountName, 0, nil)
	_, err = datastore.Put(ctx, datastore.NewIncompleteKey(ctx, collectionName, ancestorKey), &item)
	if err != nil {
		t.Fatalf("Got an error from datastore.Put: %s\n", err)
		return
	}

	var s Foo
	query := datastore.NewQuery(collectionName).Ancestor(ancestorKey).Order("-Date").Limit(1)
	_, err = query.Run(ctx).Next(&s)
	if err == datastore.Done {
		t.Errorf("No record stored. Expected %v", item)
		return
	}
	if err != nil {
		t.Errorf("Failed to fetch record: %v", err)
		return
	}
	if !reflect.DeepEqual(s, item) {
		t.Errorf("Records don't match.\nGot:  %v\nWant: %v", s, item)
	}
}

To run this, install the go_appengine package, put this in a directory in $GOPATH and run goapp test, output is:

2016/01/03 19:48:46 appengine: not running under devappserver2; using some default configuration
INFO     2016-01-04 03:48:47,077 devappserver2.py:769] Skipping SDK update check.
WARNING  2016-01-04 03:48:47,077 devappserver2.py:785] DEFAULT_VERSION_HOSTNAME will not be set correctly with --port=0
WARNING  2016-01-04 03:48:47,106 simple_search_stub.py:1126] Could not read search indexes from /var/folders/4j/2s8jcbwd2yx7xxjvjmmp76mm0000gn/T/appengine.testapp.jshirley/search_indexes
INFO     2016-01-04 03:48:47,109 api_server.py:205] Starting API server at: http://localhost:51080
INFO     2016-01-04 03:48:47,113 dispatcher.py:197] Starting module "default" running at: http://localhost:51081
INFO     2016-01-04 03:48:47,115 admin_server.py:116] Starting admin server at: http://localhost:51082
INFO     2016-01-04 03:48:48,005 datastore_stub_index.py:301] Updating /private/var/folders/4j/2s8jcbwd2yx7xxjvjmmp76mm0000gn/T/appengine-aetest881550037/index.yaml
INFO     2016-01-04 03:48:48,116 api_server.py:648] Applying all pending transactions and saving the datastore
INFO     2016-01-04 03:48:48,116 api_server.py:651] Saving search indexes
--- FAIL: TestScheduleCraziness (2.92s)
        item_test.go:37: No record stored. Expected {admin@gmail.com}
FAIL
exit status 1
FAIL    bitbucket.org/jsshirley/weird   2.928s

As soon as I posted this gist, I realized the errors of my ways.

Date is not indexed (in fact, I took it out to repro this case) and Order(...) will fail on any unindexed value.