I've got a Product POJO that looks like.
#PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Product extends AbstractModel {
#Persistent
private String name;
#Persistent
private Key homePage;
#Persistent
private Boolean featured;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Key getHomePage() {
return homePage;
}
public void setHomePage(Key homePage) {
this.homePage = homePage;
}
public boolean isFeatured() {
return featured;
}
public void setFeatured(Boolean featured) {
this.featured = featured;
}
}
My DataStore is currently completely empty.
I'd like to retrieve all homePage keys where featured is true for the Product.
I'm trying
PersistenceManager persistenceManager = getPersistenceManager();
Query query = persistenceManager.newQuery("SELECT homePage FROM " + getModelClass());
query.setFilter("featured == true");
List<Key> productPageKeys = (List<Key>) query.execute();
However this is giving me a null pointer error. How should I be constructing this query?
Cheers,
Peter
To do a projection, you would do something like
Query q = pm.newQuery("SELECT myField FROM mydomain.MyClass WHERE featured == true");
List<String> results = (List<String>)q.execute();
where String is the type of my field. Any basic JDO documentation would define that.
Internally GAE/J will retrieve the Entity, and then in the post-processing before returning it to the user it is manipulated into the projection you require.
As Nick pointed out in the other reply, this gives no performance gain over doing it yourself ... but then the whole point of a standard persistence API is to shield you from such datastore-specifics of having to do such extraction; it's all provided out of the box.
Entities are stored as serialized blobs of data in the datastore, so it's not possible to retrieve and return a single field from an entity. You need to fetch the whole entity, and extract the field you care about yourself.
Related
I try to use objectify on google app engine standart environment and get exception. My classes look like this:
#Entity
public class Company {
#Id Long id;
#Index String companyName;
public Company() {
}
public Company(Long id, String companyName) {
this.id=id;
this.companyName = companyName;
}
}
#Entity
public class CompanyProject {
#Id Long id;
#Index String projectName;
#Parent Key<Company> owner;
public String cost;
public CompanyProject() {
}
public CompanyProject(long userId, String projectName) {
this();
this.projectName = projectName;
owner = Key.create(Company.class, userId); // Creating the Ancestor key
}
}
When I query data like this:
Key<Company> theUser = Key.create(Company.class, 1);
Iterable<CompanyProject> projects = ObjectifyService.ofy().load().type(CompanyProject.class).ancestor(theUser).order("projectName").list();
I get exception
com.google.cloud.datastore.DatastoreException: no matching index found. recommended index is:
- kind: CompanyProject
ancestor: yes
properties:
- name: projectName
Without order("projectName") query works just fine. Removed all entities of this kind from datastore, than added new, still get this exception. I use Gradle, not Maven if this matters. Maybe should be extra build step to create indexes or smth.
You are filtering on multiple properties (well, ancestor counts as 'another property') so you need a multi-property index defined in datastore-indexes.xml.
See the official documentation: https://cloud.google.com/appengine/docs/standard/java/config/indexconfig
This has baffled me for a while now and I cannot seem to get the grasp of it. I'm using Cell Value Factory to populate a simple one column table and it does not populate in the table.
It does and I click the rows that are populated but I do not see any values in them- in this case String values. [I just edited this to make it clearer]
I have a different project under which it works under the same kind of data model. What am I doing wrong?
Here's the code. The commented code at the end seems to work though. I've checked to see if the usual mistakes- creating a new column instance or a new tableview instance, are there. Nothing. Please help!
//Simple Data Model
Stock.java
public class Stock {
private SimpleStringProperty stockTicker;
public Stock(String stockTicker) {
this.stockTicker = new SimpleStringProperty(stockTicker);
}
public String getstockTicker() {
return stockTicker.get();
}
public void setstockTicker(String stockticker) {
stockTicker.set(stockticker);
}
}
//Controller class
MainGuiController.java
private ObservableList<Stock> data;
#FXML
private TableView<Stock> stockTableView;// = new TableView<>(data);
#FXML
private TableColumn<Stock, String> tickerCol;
private void setTickersToCol() {
try {
Statement stmt = conn.createStatement();//conn is defined and works
ResultSet rsltset = stmt.executeQuery("SELECT ticker FROM tickerlist order by ticker");
data = FXCollections.observableArrayList();
Stock stockInstance;
while (rsltset.next()) {
stockInstance = new Stock(rsltset.getString(1).toUpperCase());
data.add(stockInstance);
}
} catch (SQLException ex) {
Logger.getLogger(WriteToFile.class.getName()).log(Level.SEVERE, null, ex);
System.out.println("Connection Failed! Check output console");
}
tickerCol.setCellValueFactory(new PropertyValueFactory<Stock,String>("stockTicker"));
stockTableView.setItems(data);
}
/*THIS, ON THE OTHER HAND, WORKS*/
/*Callback<CellDataFeatures<Stock, String>, ObservableValue<String>> cellDataFeat =
new Callback<CellDataFeatures<Stock, String>, ObservableValue<String>>() {
#Override
public ObservableValue<String> call(CellDataFeatures<Stock, String> p) {
return new SimpleStringProperty(p.getValue().getstockTicker());
}
};*/
Suggested solution (use a Lambda, not a PropertyValueFactory)
Instead of:
aColumn.setCellValueFactory(new PropertyValueFactory<Appointment,LocalDate>("date"));
Write:
aColumn.setCellValueFactory(cellData -> cellData.getValue().dateProperty());
For more information, see this answer:
Java: setCellValuefactory; Lambda vs. PropertyValueFactory; advantages/disadvantages
Solution using PropertyValueFactory
The lambda solution outlined above is preferred, but if you wish to use PropertyValueFactory, this alternate solution provides information on that.
How to Fix It
The case of your getter and setter methods are wrong.
getstockTicker should be getStockTicker
setstockTicker should be setStockTicker
Some Background Information
Your PropertyValueFactory remains the same with:
new PropertyValueFactory<Stock,String>("stockTicker")
The naming convention will seem more obvious when you also add a property accessor to your Stock class:
public class Stock {
private SimpleStringProperty stockTicker;
public Stock(String stockTicker) {
this.stockTicker = new SimpleStringProperty(stockTicker);
}
public String getStockTicker() {
return stockTicker.get();
}
public void setStockTicker(String stockticker) {
stockTicker.set(stockticker);
}
public StringProperty stockTickerProperty() {
return stockTicker;
}
}
The PropertyValueFactory uses reflection to find the relevant accessors (these should be public). First, it will try to use the stockTickerProperty accessor and, if that is not present fall back to getters and setters. Providing a property accessor is recommended as then you will automatically enable your table to observe the property in the underlying model, dynamically updating its data as the underlying model changes.
put the Getter and Setter method in you data class for all the elements.
I'm learning Google App Engine + Google Cloud Endpoints + Objectify and I'm trying to understand how to create REST API which will let each particular user to save his data in the cloud.
My current struggle is how to store Entity owned by the User (com.google.appengine.api.users.User)?
So far I have endpoint:
#ApiMethod(name = "saveBook")
public void saveBook(Book book, User user) throws OAuthRequestException, IOException {
if (user == null) {
throw new OAuthRequestException("User is not authorized");
}
ofy().save()
.entity(BookRecord.fromBook(user, book))
.now();
}
Entity (in this context let's assume that User wrote the book):
#Entity
public class BookRecord {
#Parent
private Key<User> user;
#Id
private String id;
#Index
private String name;
public static BookRecord fromBook(User user, Book book) {
return new BookRecord(
Key.create(user),
book.getId(),
book.getName()
);
}
public BookRecord() {
}
private BookRecord(Key<User> user, String id, String name, String author) {
this.user = user;
this.id = id;
this.name = name;
this.author = author;
}
}
Problem arises from User not being an Entity, so I can't really use this solution and use User as #Parent directly. What is the general solution to solve this problem and store data owned by User?
Create your own User entity, this is where you'll store custom information/preferences for your app (as I suggested here).
You'll be using getUserId() to tie it with the Google user, and from there on you're free to use it as needed.
My application is using SQLServer and JPA2 in the backend. App makes use of a timestamp column (in the SQLServer sense, which is equivalent to row version see here) per entity to keep track of freshly modified entities. NB SQLServer stores this column as binary(8).
Each entity has a respective timestamp property, mapped as #Lob, which is the way to go for binary columns:
#Lob
#Column(columnDefinition="timestamp", insertable=false, updatable=false)
public byte[] getTimestamp() {
...
The server sends incremental updates to mobile clients along with the latest database timestamp. The mobile client will then pass the old timestamp back to the server on the next refresh request so that the server knows to return only fresh data. Here's what a typical query (in JPQL) looks like:
select v from Visit v where v.timestamp > :oldTimestamp
Please note that I'm using a byte array as a query parameter and it works fine when implemented in JPQL this way.
My problems begin when trying to do the same using the Criteria API:
private void getFreshVisits(byte[] oldVersion) {
EntityManager em = getEntityManager();
CriteriaQuery<Visit> cq = cb.createQuery(Visit.class);
Root<Visit> root = cq.from(Visit.class);
Predicate tsPred = cb.gt(root.get("timestamp").as(byte[].class), oldVersion); // compiler error
cq.where(tsPred);
...
}
The above will result in compiler error as it requires that the gt method used strictly with Number. One could instead use the greaterThan method which simply requires the params to be Comparable and that would result in yet another compiler error.
So to sum it up, my question is: how can I use the criteria api to add a greaterThan predicate for a byte[] property? Any help will be greatly appreciated.
PS. As to why I'm not using a regular DateTime last_modified column: because of concurrency and the way synchronization is implemented, this approach could result in lost updates. Microsoft's Sync Framework documentation recommends the former approach as well.
I know this was asked a couple of years back but just in case anyone else stumbles upon this.. In order to use a SQLServer rowver column within JPA you need to do a couple of things..
Create a type that will wrap the rowver/timestamp:
import com.fasterxml.jackson.annotation.JsonIgnore;
import javax.xml.bind.annotation.XmlTransient;
import java.io.Serializable;
import java.math.BigInteger;
import java.util.Arrays;
/**
* A RowVersion object
*/
public class RowVersion implements Serializable, Comparable<RowVersion> {
#XmlTransient
#JsonIgnore
private byte[] rowver;
public RowVersion() {
}
public RowVersion(byte[] internal) {
this.rowver = internal;
}
#XmlTransient
#JsonIgnore
public byte[] getRowver() {
return rowver;
}
public void setRowver(byte[] rowver) {
this.rowver = rowver;
}
#Override
public int compareTo(RowVersion o) {
return new BigInteger(1, rowver).compareTo(new BigInteger(1, o.getRowver()));
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
RowVersion that = (RowVersion) o;
return Arrays.equals(rowver, that.rowver);
}
#Override
public int hashCode() {
return Arrays.hashCode(rowver);
}
}
The key here is that it implement Comparable if you want to use it in calculations (which you definitely do)..
Next create a AttributeConverter that will move from a byte[] to the class you just made:
import javax.persistence.AttributeConverter;
import javax.persistence.Converter;
/**
* JPA converter for the RowVersion type
*/
#Converter
public class RowVersionTypeConverter implements AttributeConverter<RowVersion, byte[]> {
#Override
public byte[] convertToDatabaseColumn(RowVersion attribute) {
return attribute != null ? attribute.getRowver() : null;
}
#Override
public RowVersion convertToEntityAttribute(byte[] dbData) {
return new RowVersion(dbData);
}
}
Now let's apply this RowVersion attribute/type to a real world scenario. Let's say you wanted to find all Programs that have changed on or before some point in time.
One straightforward way to solve this would be to use a DateTime field in the object and timestamp column within db. Then you would use 'where lastUpdatedDate <= :date'.
Suppose that you don't have that timestamp column or there's no guarantee that it will be updated properly when changes are made; or let's say your shop loves SQLServer and wants to use rowver instead.
What to do? There are two issues to solve.. one how to generate a rowver and two is how to use the generated rowver to find Programs.
Since the database generates the rowver, you can either ask the db for the 'current max rowver' (a custom sql server thing) or you can simply save an object that has a RowVersion attribute and then use that object's generated RowVersion as the boundary for the query to find the Programs changed after that time. The latter solution is more portable is what the solution is below.
The SyncPoint class snippet below is the object that is used as a 'point in time' kind of deal. So once a SyncPoint is saved, the RowVersion attached to it is the db version at the time it was saved.
Here is the SyncPoint snippet. Notice the annotation to specify the custom converter (don't forget to make the column insertable = false, updateable = false):
/**
* A sample super class that uses RowVersion
*/
#MappedSuperclass
public abstract class SyncPoint {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
// type is rowver for SQLServer, blob(8) for postgresql and h2
#Column(name = "current_database_version", insertable = false, updatable = false)
#Convert(converter = RowVersionTypeConverter.class)
private RowVersion currentDatabaseVersion;
#Column(name = "created_date_utc", columnDefinition = "timestamp", nullable = false)
private DateTime createdDate;
...
Also (for this example) here is the Program object we want to find:
#Entity
#Table(name = "program_table")
public class Program {
#Id
private Integer id;
private boolean active;
// type is rowver for SQLServer, blob(8) for postgresql and h2
#Column(name = "rowver", insertable = false, updatable = false)
#Convert(converter = RowVersionTypeConverter.class)
private RowVersion currentDatabaseVersion;
#Column(name = "last_chng_dt")
private DateTime lastUpdatedDate;
...
Now you can use these fields within your JPA criteria queries just like anything else.. here is a snippet that we used inside a spring-data Specifications class:
/**
* Find Programs changed after a synchronization point
*
* #param filter that has the changedAfter sync point
* #return a specification or null
*/
public Specification<Program> changedBeforeOrEqualTo(final ProgramSearchFilter filter) {
return new Specification<Program>() {
#Override
public Predicate toPredicate(Root<Program> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
if (filter != null && filter.changedAfter() != null) {
// load the SyncPoint from the db to get the rowver column populated
SyncPoint fromDb = synchronizationPersistence.reload(filter.changedBeforeOrEqualTo());
if (fromDb != null) {
// real sync point made by database
if (fromDb.getCurrentDatabaseVersion() != null) {
// use binary version
return cb.lessThanOrEqualTo(root.get(Program_.currentDatabaseVersion),
fromDb.getCurrentDatabaseVersion());
} else if (fromDb.getCreatedDate() != null) {
// use timestamp instead of binary version cause db doesn't make one
return cb.lessThanOrEqualTo(root.get(Program_.lastUpdatedDate),
fromDb.getCreatedDate());
}
}
}
return null;
}
};
}
The specification above works with both the binary current database version or a timestamp.. this way I could test my stuff and all the upstream code on a database other than SQLServer.
That's it really: a) type to wrap the byte[] b) JPA converter c) use attribute in query.
I have a Java model similar to:
public class Country {
#Id private String id;
private CurrencyId currencyId;
private List<Province> provinceList;
...
}
public class Province {
#Id private String id;
private Gobernor gobernorId;
private List<City> cityList;
...
}
public class City {
#Id private String id;
private String name;
...
}
I want to store that data using objectify. However, as Country data might change, I also want to store the date the Country data has been stored, so I think I should store an entity such as:
public class CountryListEntity {
#Id private String id;
private List<Country> countryList;
private Date storeDate;
}
Note I will only have one entity of kind CountryListEntity with the Id "root", if I can store it like that. I know very little about both how google apps stores data and how objectify works. I've tried many combinations of #Embedded, but I got many errors, i.e.
Cannot place array or collection properties inside #Embedded arrays or collections
Can anyone tell me how to define these classes? A snippet of the code needed to store and retrieve this "root" entity, would be highly appreciated!
#Embedded collections are transformed into a series of collection fields in the
low-level Entity. That's why one level embedding is all you can do.
If you are going to store/load all data at once and if your entities are as simple as the ones in your example you can put #Serialized annotation for your lists inside #Embedded lists.
You can find out more from this discussion.
The problem with this approach is that your low-level embeddings won't be able to be indexed.
public class CountryListEntity {
#Id private String id;
#Embedded
private List<Country> countryList;
private Date storeDate;
}
public class Country implements Serializable {
private String id;
private CurrencyId currencyId;
#Serialized
private List<Province> provinceList;
// ...
}
public class Province implements Serializable {
private String id;
private Gobernor gobernorId;
#Serialized
private List<City> cityList;
// ...
}
public class City implements Serializable {
private String id;
private String name;
// ...
}