App Engine Datastore query result is returned randomly

Question

As an exercise to better understand how the Datastore works, I have created within the HttpServlet class of my project a datastore with several entities and relative properties. After creating and populating the datastore, I immediately query it in order to return a Json object back to the Client so it can dynamically update its UI.

It works, but I have a problem: after the first time I query the datastore, I always receive a result ordered differently, often with duplicates of the same items.

This is my HttpServlet code:

public class MyServlet extends HttpServlet {
    ArrayList<Tour> m_tours = new ArrayList<Tour>();
    Key tourKey;
    DatastoreService datastore;
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {
        //nothing special here
    }

    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        populateDatastore();

        String asyncMessage = req.getParameter("order");
        if(asyncMessage.equals("tours")){
            m_tours = getTours();
        }
        if(asyncMessage.equals("selectTour")){

        }

        Tours tours = new Tours(m_tours);
        resp.setContentType("application/json");
        PrintWriter out = resp.getWriter();
        out.print(new Gson().toJson(tours));
        out.flush();
    }
    private DatastoreService populateDatastore(){

    datastore = DatastoreServiceFactory.getDatastoreService();
    tourKey = KeyFactory.createKey("availabletours", "tours"); //parent

    Entity tour = new Entity("tour", tourKey);
    tour.setProperty("tourname", "Tour0");
    tour.setProperty("tourinfo", "info0");
    datastore.put(tour);

    Entity tour1 = new Entity("tour1", tourKey);
    tour1.setProperty("tourname", "Tour 1");
    tour1.setProperty("tourinfo", "info 1");
    datastore.put(tour1);

    //..... and so on

    return datastore;
}

private ArrayList<Tour> getTours(){
    ArrayList<Tour> toursArray = new ArrayList<Tour>();

    Query query = new Query(tourKey);
    List<Entity> tourss = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(9));
    for (Entity t : tourss) {
        String tourname=t.getProperty("tourname").toString();
        String tourinfo=t.getProperty("tourinfo").toString();
        Tour ts = new Tour(tourname,tourinfo);
        toursArray.add(ts);
    }

    return toursArray;
    }
}

What am I doing wrong? What is the best way to manage a datastore?

* EDIT *

After uniforming the Entity objects to the same type and setting an ancestor query, as suggested by @Jeff Deskins:

    datastore = DatastoreServiceFactory.getDatastoreService();
    tourKey = KeyFactory.createKey("availabletours", "tours"); //parent

    Entity tour = new Entity("tour", tourKey);
    tour.setProperty("tourname", "Tour0");
    tour.setProperty("tourinfo", "info0");
    datastore.put(tour);

    Entity tour1 = new Entity("tour", tourKey);
    tour1.setProperty("tourname", "Tour 1");
    tour1.setProperty("tourinfo", "info 1");
    datastore.put(tour1);

    //... ...  same way with the others

I set an ancestor query:

    Query query = new Query("tour").setAncestor(tourKey);
    List<Entity> tourss = datastore.prepare(query).asList(FetchOptions.Builder.withLimit(9));
    for (Entity t : tourss) {
        String tourname=t.getProperty("tourname").toString();
        String tourinfo=t.getProperty("tourinfo").toString();
        Tour ts = new Tour(tourname,tourinfo);
        toursArray.add(ts);
    }

And this is the result I get:

First time: Second time:

EDIT 2: Answer - Apart from performing an ancestor query, I also needed to apply a Sort to the Query. Therefore the solution is to add a property to each entity:

  Entity tour = new Entity("tour", tourKey);
        tour.setProperty("tourname", "Tour 1");
        tour.setProperty("tourinfo", "info 1");
        tour.setProperty("order","0");
  Entity tour = new Entity("tour", tourKey);
        tour.setProperty("tourname", "Tour 2");
        tour.setProperty("tourinfo", "info 2");
        tour.setProperty("order","1");

and then apply a sort filter to the query:

    ArrayList<Tour> toursArray = new ArrayList<Tour>();

    Query query = new Query("tour").setAncestor(tourKey).addSort("order");
    List<Entity> tourss = new ArrayList<Entity>();
    tourss = datastore.prepare(query).asList(FetchOptions.Builder.withDefaults());

    for (Entity t : tourss) {
        String tourname=t.getProperty("tourname").toString();
        String tourinfo=t.getProperty("tourinfo").toString();
        Tour ts = new Tour(tourname,tourinfo);
        toursArray.add(ts);
    }

now the result is sorted properly

Answer 1

Your Entity objects should follow the same pattern if they are all the same type.

Creating different entities of same type with same parent key:

Entity tour1 = new Entity("Tour", tourKey);
Entity tour2 = new Entity("Tour", tourKey);

After populating and saving the above objects to the datastore, you can then query by:

Query tourQuery = new Query("Tour")
                         .setAncestor(tourKey);

The ancestor query provides strong consistency. https://cloud.google.com/appengine/docs/java/datastore/queries#Java_Ancestor_queries