The following (abstracted) code gives a StackOverflow error, I assume due to recursive referencing between Team and Member (Team contains Member, which contains Team, etc).
#Entity public class Team {
#Id public String id;
public List<Member> members;
public Team() {
this.id = UUID.randomUUID().toString();
}
}
public class Member {
#Container Team team; //removing this line prevents error, but then how to reference this member's team?
public Member() {}
}
How does one set up the annotations for this relationship properly, so that a team has references to its members, and each member has a reference to its team?
#Container causes an embedded entity to be populated with the referenced entity. If you do what you have above, when you load one of Team or Member, it will load the other as an embedded entity in it, which then load the former again inside it -> Obviously this is an infinite recursion of Teams and Members all the way to an overflow.
Assuming a Member can only be on one team. perhaps you are better setting Team to be a #Parent (Teams contain members, a member is part of a team). Keep in mind this means you'll have a sustained 1 write per second limit, although you can burst higher for short durations.
Related
I have a Course entity that contains the following field
#Index
private #Load
Ref<Student> student;
The student entity then has the field
#Index
private String matric;
I want to load all the Course entities sorted using the students matric number.
I have tried using the "." operator to get the sub field like this
ofy().load().type(Course.class).filter("course", course).order("student.matric").list();
but this return no result.
Is it possible to do this? how?
I don't think that is possible with objectify. I would let Course implement Comparable:
#Entity
public class Course implements Comparable<Course> {
.
.
.
#Override
public int compareTo(Course otherCourse) {
return this.getStudent().getMatric().compareTo(otherCourse.getStudent().getMatric());
}
}
Remove the "order" part of the Objectify load and use Collections.sort() instead:
List<Course> courses = ofy().load().type(Course.class).filter("course", course).list();
Collections.sort(courses);
There are no joins in the datastore. If you want to query your Courses by Student properties, you probably will need to denormalize the data into the Course and index it. This means changing the Student data will also require changing Courses.
As an aside: This data model is weird. Are you sure what you're calling Course isn't really an Enrollment?
Why is it not possible to unindex a list of an objectify entity?
To demonstrate the problem I made a simple example project.
I used the entity
#Entity
public class Car {
#Id String id;
#Unindex List<Passenger> passengers;
}
an the object
public class Passenger {
String name;
}
and saved it using this simple method.
public class CarFactory {
public void writeCarEntity() {
Car car = new Car();
car.setId("myCar");
List<Passenger> passengers = new LinkedList<Passenger>();
Passenger carl = new Passenger();
carl.setName("Carl");
Passenger pete = new Passenger();
pete.setName("Pete");
Passenger jeff = new Passenger();
jeff.setName("Jeff");
passengers.add(carl);
passengers.add(pete);
passengers.add(jeff);
car.setPassengers(passengers);
ObjectifyService.register(car.getClass());
ObjectifyService.ofy().save().entity(car).now();
}
}
Looking up the entity in the datastore you get this information:
Although the passengers field has the annotation #Unindex it will be indexed, as one can see in the google "Datastore". Why does the annotation #Unindex has no effect in this example???
This is unrelated to Objectify and appears to be some new quirk of the datastore. It might just be a display glitch in the UI. Is it causing problems?
With the code you posted, Objectify will call Entity.saveUnindexedProperty() on the passengers field (even without the #Unindex annotation). But even if Objectify tried to index it, historically you can't index embedded objects, so it's unclear what it means to index a list of them. Maybe Google is rolling out some new behavior and they haven't got the GUI working correctly yet? Or maybe there is a bug in their save behavior?
If you want to be a good citizen, create a simple test case with the low level API (an Entity that contains a property of type List<EmbeddedEntity>), verify that this same behavior occurs, and file a bug in the GAE issue tracker.
Let's say I have a very easy, classic setup: GAE(1.7.4) + GWT(2.5.0) Application, running on local Jetty (Development Server), using JDO for persistence.
Let's also say I have just 2 #PersistenceCapable classes: Person and Color. Every Person has exactly one favourite Color, but it does not mean that this Person owns this Color - many different Persons can have the same favourite Color. There is a limited number of well-known Colors and a Color may exist even if it is not anyone's favourite.
To model this I should use #Unowned relationship - please correct me if I am wrong:
#PersistenceCapable
public class Color { // just the most regular Entity class
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;
#Persistent
String rgb;
// getter, setter, no constructor
}
#PersistenceCapable
public class Person {
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;
#Persistent
String surname;
#Persistent
#Unowned // here is the tricky part
Color color;
// getters, setters, no constructor
}
With some simple, well-known, PersistentManager-based code, I am able to successfully create and persist an instance of a Color class. I see it in GAE Development Console -> Datastore Viewer, having nice generated Key and ID/Name of (13), and my assigned RGB.
With very similar code, I am able to create an instance of Person class (in another request), assign a pre-existing Color as his favourite color (it pre-existed, I obtained it by pm.getObjectById()) and persist it. I see it in Datastore Viewer, with my nice generated Key and ID/Name of (15) and my assigned surname, and color_key_OID of (13). This looks very promising.
But then, when I fetch the Person(15) back from the DB (simple pm.getObjectById(), no transactions), it has my assigned surname correctly, but has null instead of Color(13)! Right - the Datastore Viewer gets it ok, but my code does not.
Oh, the problematic code? "Person p = pm.getObjectById(Person.class, key);".
(side notes: I am also having the same problem with #Unowned collections (nice list of values in Datastore Viewer, but null Collection field in my code.) My JDO jars on classpath are "datanucleus-api-jdo-3.1.1.jar" and "jdo-api-3.0.1.jar" so I assume they support #Unowned. There is no problem with not-#Unowned fields. I get no exceptions upon persisting or fetching, just plain nulls as field values.)
Either mark the color to be "eagerly fetched"
#Persistent(defaultFetchGroup="true")
#Unowned
Color color
or define your own fetchgroup like this:
#FetchGroup(name="eager", members={#Persistent(name="color")})
#PersistenceCapable
public class Person {
and use it if required by specifying the group to be fetched:
PersistenceManager pm = pmf.getPersistenceManager();
pm.getFetchPlan().addGroup("eager");
I was facing the same issue in one of my #Unowned Lists. I had more other two, which the Array is fetched perfectly.
What solved this issue for me was to change the name of property for a bigger one. In your case is like change the property name from "color" to something bigger, like "myfavoritecolor".
I have the same issue what you describe. How DataNucleus said you need to describe the whole lifecycle of the objects. In my case the problem was solved forcing getting the color, from the person object, before closing the PersistenceManager with the close() function.
Remember JDO uses the lazy-load technique to get objects.
I was able to solve this problem by adding fetch groups to the query and not to persistent manager.
PersistenceManager pm = PMF.get().getPersistenceManager();
logger.info("EVENTS FETCH GROUPS : " + pm.getFetchPlan().getGroups());
/*pm.getFetchPlan().addGroup("eventFetchGroup");
pm.getFetchPlan().setMaxFetchDepth(2);*/
Query q = pm.newQuery(Event.class);
q.getFetchPlan().addGroup("eventFetchGroup");
logger.info("EVENTS FETCH GROUPS : " +q.getFetchPlan().getGroups());
q.setFilter("date >= fromDate && date <= toDate");
q.declareParameters("java.util.Date fromDate, java.util.Date toDate");
I'm using app engine datastore so I have entity like this.
#PersistenceCapable
public class Author {
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#JsonProperty("id")
#JsonSerialize(using = JsonKeySerializer.class)
#JsonDeserialize(using = JsonKeyDeserializer.class)
private Key key;
....
}
When the model is sent to view, it will serialize the Key object as an Id value. Then, if I send data back from view I want to deserialize the Id back to Key object by using JsonKeyDeserializer class.
public class JsonKeyDeserializer extends JsonDeserializer<Key> {
#Override
public Key deserialize(JsonParser jsonParser, DeserializationContext deserializeContext)
throws IOException, JsonProcessingException {
String id = jsonParser.getText();
if (id.isEmpty()) {
return null;
}
// Here is the problem because I have several entities and I can't fix the Author class in this deserializer like this.
// I want to know what class is being deserialized at runtime.
// return KeyFactory.createKey(Author.class.getSimpleName(), Integer.parseInt(id))
}
}
I tried to debug the value in deserialize's parameters but I can't find the way to get the target deserialized class. How can I solve this?
You may have misunderstood the role of KeySerializer/KeyDeserializer: they are used for Java Map keys, and not as generic identifiers in database sense of term "key".
So you probably would need to use regular JsonSerializer/JsonDeserializer instead.
As to type: it is assumed that handlers are constructed for specific types, and no extra type information is passed during serialization or deserialization process: expected type (if handlers are used for different types) must be passed during construction.
When registering general serializers or deserializers, you can do this when implementing Module, as one of the arguments is type for which (de)serializer is requested.
When defining handlers directly for properties (like when using annotations), this information is available on createContextual() callback of interface ContextualSerializer (and -Deserializer), if your handler implements it: BeanProperty is passed to specify property (in this case field with annotation), and you can access its type. This information needs to be stored to be used during (de)serialization.
EDIT: as author pointed out, I actually misread the question: KeySerializer is the class name, not annotation.
This is my first question on any of these websites so pardon my unprofessionalism.
I use playframework with SIENA module (with GAE) and I came accross the following problem:
Given 3 entities:
public class Meeting extends Model{
#Id
public Long id;
public String place;
#Owned
Many<MeetingUser> users;
.
.
.
}
public class User extends Model{
#Id
public Long id;
public String firstName;
public String lastName;
#Owned
Many<MeetingUser> meetings;
.
.
.
}
public class MeetingUser extends Model{
#Id
public Long id;
public Meeting meeting;
public User user;
.
.
.
public User getUser(){
return Model.all(User.class).filter("id", user).get();
}
public Meeting getMeeting(){
return Model.all(Meeting.class).filter("id", meeting).get();
}
}
For instance I am listing a meeting and all their users:
public static void meetingInfo(Long meetingId){
Meeting meeting = Models.all(Meeting.class).filter("id",meetingId);
List<MeetingUser> meetingusers = meeting.asList();
List<User> users = new ArrayList<User>();
for(MeetingUser mu: meetingusers){
users.add(mu.getUser());
}
render(users);
}
This is done(is there any better way here?) however when it comes to filtering (especially dynamic filtering for many many fields) I can not use the Query's filter method on the MeetingUser as I need to filter on a MeetingUser's field's field (firstName). The same problem arise for ordering. I need the solution for both problems.
I hope my problem is clear and I appreciate any kind of help here.
Remember that you are in GAE which is a NoSQL DB.
So you can't do Join request as in RDBMS.
Yet, this is not really the pb you have so this was just to be sure you are aware of it ;)
So if you want to find the person having given firstname in a given meeting, can you try the following:
List<MeetingUser> meetingusers = meeting.users.asQuery().filter("firstname", "XXX");
(you can also order)
Nevertheless, knowing that you can't join, remember that you can't write a query searching for a meeting in which there are users whose firstname is XXX as it would require some joins and it doesn't exist in GAE. In this case, you need to change your model following NoSQL philosophy but this is another subject
regards
Let's try to give a way to do what you want...
Your relation is a Many-to-Many which is always the worst case :)
You want to filter Meeting by User's firstname.
It requires a join request which is not possible in GAE. In this case, you must change your model by denormalizing it (sometimes use redundancy also) and manage the join by yourself. Actually, you must do the job of the RDBMS by yourself. It seems overkill but in fact, it's quite easy. The only drawback is that you must perform several requests to the DB. NoSQL means No Schema (& No Join) so there are a few drawbacks but it allows to scale and to manage huge data load... it depends on your needs :)
The choice you did to create the MeetingUser which is a "joined" table and a kind of denormalization is good in GAE because it allows to manage the join yourself.
Solution:
// fetch users by firstname
List<User> users = users.all().filter("firstName", "John").fetch();
// fetch meetingusers associated to these users (verify the "IN" operator works because I didn't use that for a long time and don't remember if it works with this syntax)
List<MeetingUser> meetingusers = MeetingUser.all().filter("user IN", users);
// now you must fetch the whole meeting because in MeetingUser, only the Meeting ID is stored (other fields are Null or O)
List<Meeting> meetings = new ArrayList<Meeting>()
for(MeetingUsers mu:meetingusers) {
meetings.add(meetingusers.meeting);
}
// use the batch feature to fetch all objects
Meeting.batch(Meeting.class).get(meetings);
// you have your meetings
Hope this helps!