I have two classes InvitedPerson
and Flight
with a one to one relationship with each other. Here is how they are annotated.
public class InvitedTech{
...
@OneToOne(mappedBy="invitedTech", cascade = CascadeType.ALL, fetch=FetchType.LAZY)
public Flight flight;
@OneToOne(mappedBy="invitedTech", cascade = CascadeType.ALL, fetch=FetchType.LAZY)
public Hotel hotel;
...
}
public class Flight{
...
@OneToOne
@JoinColumn(name="invitedTechId", nullable=false)
public InvitedTech invitedTech;
...
}
As you can see Flight
is the owner of the relationship and InvitedTech
is the other side of this bidirectional relationship. InvitedTech also has a OneToOne
relationship with Hotel
Now, when I write a simple query to fetch all flights, it triggers three queries in total. 1st which gets me the results, but fires 2 additional queries.
List<Flight> flg = JPA.em().createQuery("SELECT flg from Flight flg").getResultList();
Why are query 2&3 being executed even though I have set FetchType=Lazy. I am not accessing Hotel Information. And Flight should not be queries again as the first query returns the data.
After some playing around when I remove mappedBy
attribute from both the annotations, those 2 addition queries don't get executed(i.e only 1st gets executed).
Why does the mappedBy
attribute cause additional queries to be executed even though FetchType=Lazy
. Is there a way to stop this?
I believe this is due to one of Hibernate's idiosyncrasies:
non-optional one-to-one relationships are eagerly loaded regardless of whether they are mapped as Lazy.
The reasoning behind this is that as the engine has to look in the association table anyway - to determine whether it should set the association as a proxy or as null - then it may as well load the associated entity anyway.
I have experienced this myself and as far as I know the only way round it is to mark the relationship with optional=false which tells Hibernate it can always set a proxy.
If the relationship is optional then the only other option seems to be byte code instrumentation.
See also:
https://community.jboss.org/wiki/SomeExplanationsOnLazyLoadingone-to-one