I have recently started to use Spring Data MongoDB and I wonder if there is any way to avoid writing entities' attributes explicitly as they are stored in the database. For example, given the following class representing a MongoDB collection:
public class Employee {
#Id
public String id;
private double salary;
...
}
If I want to make a query using MongoTemplate like:
public List findEmployeeBySalaryRange(double salary) {
Query query = new Query();
query.addCriteria(Criteria.where("salary").lt(salary));
...
}
I would like to avoid writing "salary", since that will make the code harder to maintain in the future in case the field name changes. I am thinking of something like getting the field name from the class attribute, but I am not quite sure how. Is there a way to do it? I have looked into the documentation but did not find anything related unless I missed it.
Thanks in advance.
You may create a Utility Class to store all database field names, use #Field annotation on field with constant from that class and use that constant in query to avoid error prone hardcoded Strings.
In Employee Model
#Field(DbFields.SALARY)
private double salary;
In Query,
query.addCriteria(Criteria.where(DbFields.SALARY).lt(salary));
In DbFields Utility class
public static final String SALARY = "salary";
Related
I know that generally, we need to do something similar to this for getting a document back from mongodb in spring data:
Define a class and annotate it with #Document:
#Document ("persons")
public class Person
Use MongoTemplete:
mongoOps.findById(p.getId(), Person.class);
The problem is that in runtime I don't know the class type of the document, I just have its string collection name and its string Id. How is it possible to retrieve the document using SpringData? Something like this:
db.myCollectionName.findOne({_id: myId})
The result object type is not a concern, it can be even an object, I just want to map it to a jackson JsonNode.
A possible workaround for this you can use the aggregate function of mongooperation like this
AggregationResults<Object> aggResults = mongoOps.aggregate(newAggregation(match(Criteria.where("_id").is(myId)) ,
myCollectionName, Object.class);
return aggResults.getUniqueMappedResult();
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?
I am using Objectify to manage GAE Datastore for my GWT app. The problem is that I am not using queries properly and I get UmbrellaExceptions as per below:
Caused by: java.lang.RuntimeException: Server Error: java.lang.String cannot be cast to java.lang.Number
at com.google.web.bindery.requestfactory.shared.Receiver.onFailure(Receiver.java:44)
Say that I have a class Box with a unique field String id. I want to get the Box object whose id == "cHVQP6zZiUjM"
This is how I do it now:
public Box getBox(String boxId)
{
Objectify ofy = ObjectifyService.begin();
Query<Box> q=ofy.query(Box.class).filter("id",boxId);
Box targetBox = q.get();
return targetBox;
}
#Entity
public class Box extends DatastoreObject{
private String id;
private String title;
}
I tried doing this with ofy.load() but that method is not defined in my class Objectify (I don't know why).
Your key is encoded. Try using:
Box targetBox = ofy.get(Box.class, KeyFactory.stringToKey(boxId));
To decode your key.
The short answer: You are missing the #Id annotation in your entity.
The long answer: Id fields are special in the datastore. The id is not a real property, but rather a part of the Key that identifies the entity. You can't really filter on id fields, but you can filter on a special field called __key__. Objectify is somewhat clever about letting you filter by the id field and converting this to a __key__ filter under the covers, but it can't do it if you don't annotate the entity properly!
Actually I'm a little confused because Objectify shouldn't let you register the entity without an #Id field.
By the way, there are two sections of the documentation: Objectify4 (release coming soon) and Objectify3. Since you're using Ofy3, there is no load() method.
Another thing: Get-by-key operations are strongly preferred to queries when the operations are equivalent (as they are in your example).
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!
Some Background
I have a game database with a table called Games that has multiple attributes and one called Genres. The Genres attribute is defined as an integer[] in PostgreSQL. For the sake of simplicity, I'm not using any foreign key constraints, but essentially each integer in this array is a foreign key constraint on the id attribute in the Genres table. First time working with the NetBeans Master/Detail Sample Form and Java persistence and it's been working great so far except for 1 thing. I get this error when the program tries to display a column that has a 1-dimensional integer array. In this example, the value is {1, 11}.
Exception Description: The object [{1,11}], of class [class org.postgresql.jdbc3.Jdbc3Array], from mapping [oracle.toplink.essentials.mappings.DirectToFieldMapping[genres-->final.public.games.genres]] with descriptor [RelationalDescriptor(finalproject.Games --> [DatabaseTable(final.public.games)])], could not be converted to [class [B].
Exception [TOPLINK-3002] (Oracle TopLink Essentials - 2.0.1 (Build b09d-fcs (12/06/2007))): oracle.toplink.essentials.exceptions.ConversionException
My Research
From what I've been able to read, it looks like PostgreSQL arrays need something special done to them before you can display and edit them in this template. By default, the sample form uses TopLink Essentials (JPA 1.0) as its persistence library, but I can also use Hibernate (JPA 1.0).
Here is the code that needs to be changed in some way. From the Games.java file:
#Entity
#Table(name = "games", catalog = "final", schema = "public")
#NamedQueries({
// omitting named queries
#NamedQuery(name = "Games.findByGenres", query = "SELECT g FROM Games g WHERE g.genres = :genres")
})
public class Games implements Serializable {
#Transient
private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);
private static final long serialVersionUID = 1L;
// omitting other attributes
#Column(name = "genres")
private Serializable genres;
// omitting constructors and other getters/setters
public Serializable getGenres() {
return genres;
}
public void setGenres(Serializable genres) {
Serializable oldGenres = this.genres;
this.genres = genres;
changeSupport.firePropertyChange("genres", oldGenres, genres);
}
} // end class Games
Here are also some of the sites that might have the solution that I'm just not understanding:
https://forum.hibernate.org/viewtopic.php?t=946973
http://blog.xebia.com/2009/11/09/understanding-and-writing-hibernate-user-types/
// omitted hyperlink due to user restriction
Attempted Solutions
I'm able to get the data to display if I change the type of genres to String, but it is immutable and I cannot edit it. This is what I changed to do this:
#Column(name = "genres")
private String genres;
public String getGenres() {
return genres;
}
public void setGenres(String genres) {
String oldGenres = this.genres;
this.genres = genres;
changeSupport.firePropertyChange("genres", oldGenres, genres);
}
I also attempted to create a UserType file for use with Hibernate (JPA 1.0), but had no idea what was going wrong there.
I also attempted to use the #OneToMany and other tags, but these aren't working probably because I'm not using them properly.
What I'm Looking For
There has to be a simple way to get this data to display and make it editable, but since I'm completely new to persistence, I have no idea what to do.
The effort put into your question shows. Unfortunately JPA does not currently support PostgreSQL arrays. The fundamental problem is that arrays are not frequently used in many other databases frequently and so heavy reliance on them is somewhat PostgreSQL specific. Thus you can expect that general cross-db persistence API's are not generally going to support them well if at all. JPA is no exception, having currently no support for PostgreSQL arrays.
I have been looking at writing my own persistence API in Java that would support arrays, but it hasn't happened yet, would be PostgreSQL-only when written, and would be based on a very different principle than JPA and friends.