How to load OneToMany Collections data in response using FetchType.LAZY? - spring

Question

I have created a sample jHipster sample app( url: http://jhipster.github.io/creating_an_app.html), using entity sub-generator I have created an Event entity which has OneToMany relationship with EventImages, EventTickets and EventQuestions when I retrieve all(app running in local machine, the api url: http://127.0.0.1:8080/api/events ) events I couldn't find EventImages, EventTickets and EventQuestions data in response.
Event Entity
#Entity
#Table(name = "JHI_EVENT")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class Event implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
/* other fields */
#OneToMany(mappedBy = "event")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonIgnore
private Set<EventTicket> eventTickets = new HashSet<>();
#OneToMany(mappedBy = "event")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonIgnore
private Set<EventImage> eventImages = new HashSet<>();
#OneToMany(mappedBy = "event")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonIgnore
private Set<EventQuestion> eventQuestions = new HashSet<>();
/* getter and setters */
}
EventImages entity
#Entity
#Table(name = "JHI_EVENTIMAGE")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class EventImage implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#Column(name = "image_url")
private String imageUrl;
#ManyToOne
private Event event;
/* getters and setters */
}
similarly, EventTickets and EventQuestions entities.
After some research i found that i need to remove #JsonIgnore annotation to load OneToMany Collections data using lazy fetch, the response i got is null for EventImage, EventTicket and EventQuestions, as below.
[ {
"id": 1,
"title": "First Event",
"eventVenue": "xyz",
"startDate": "2015-05-28T10:10:00Z",
"endDate": "2015-06-20T10:10:00Z",
"eventTickets": null,
"eventImages": null,
"eventQuestions": null
} ]
Then I found I need use #JsonManagedReference and #JsonBackReference on parent/child relation, but need to use fetch = Fetch.EAGAR (I want load OneToMany Collections when I set FetchType.LAZY which is default, as an when Event entity is called).
Event entity when I used #JsonManagedReference
#Entity
#Table(name = "JHI_EVENT")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class Event implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
/* other fields */
#OneToMany(mappedBy = "event", fetch = FetchType.EAGER)
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonManagedReference
private Set<EventTicket> eventTickets = new HashSet<>();
#OneToMany(mappedBy = "event", fetch = FetchType.EAGER)
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonManagedReference
private Set<EventImage> eventImages = new HashSet<>();
#OneToMany(mappedBy = "event", fetch = FetchType.EAGER)
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
#JsonManagedReference
private Set<EventQuestion> eventQuestions = new HashSet<>();
/* getter and setters */
}
EventImage entity when I used #JsonBackReference
#Entity
#Table(name = "JHI_EVENTIMAGE")
#Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class EventImage implements Serializable {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#Column(name = "image_url")
private String imageUrl;
#ManyToOne
#JsonBackReference
private Event event;
/* getters and setters */
}
How to load OneToMany Collections lazily i.e. EventImages, EventTickets and EventQuestions in response when Event entity requested i.e http://127.0.0.1:8080/api/events REST call is made.
Thanks

Answer 1

I don't actually understand your question, but I try to explain how JPA works.
As you mentioned, using JPA you can either load collections LAZILY or EAGERLY.
LAZY means that you do not load the collection from the beginning. The collection is only loaded when you access the collection (this only works in the same transaction where the entity is loaded or attached).
EAGER means that the collection is loaded from the beginning (as the entity itself is loaded).
So if you want to provide the collection by the REST service, then you have to load the collections during the transaction.
This can be done in several ways:
One way is to define the FetchType of the collection to EAGER
Another way is to LAZY load the collection and after loading the entity, access the collection (for example by calling size() -> event.getEventImages().size();)
Another way is to load the entity and the collection with a JPQL-Query (SELECT e FROM Event JOIN FETCH e.eventImages ...)
There are even more ways to achieve this depending on the JPA implementation you are using
So, if I understood your question rigth, then you could Annotate your Spring-Data-DAO-Find-Method with
#Query("SELECT e FROM Event JOIN FETCH e.eventTickets, JOIN FETCH e.eventImages, JOIN FETCH e.eventQuestions")