Given this entity:
#Entity
public class Person {
#Id
#GeneratedValue
private Long id;
#OneToMany(fetch = FetchType.LAZY)
private List<Role> roles;
...other attributes...
}
I'd like to select some instances using a TypedQuery<Person>.
The nature† of my query is such that it is strictly required that only instances of Person are to be selected on a first shot; only after that first query should the other relationships be queried too.
Clearly the opposite of FetchType.EAGER. But FetchType.LAZY is just a hint, the JPA implementation being allowed to ignore it.
How can I force lazy fetch?
I'm using OpenLiberty (based on EclipseLink). I'd rather use some general approach that will work on any JPA container; but maybe this is not possible.
†The nature of my query is that it is being limited by javax.persistence.Query::setMaxResults(50), so that I want to get only the first 50 Persons, regardless of how many Roles each one has. As it is now, I get the first 50 Person—Role, and after JPA sifts through retrieved data I'm left with about 32 Persons.
Related
I am using NHibernate in a web application I'm building. The user can subscript to zero or more mailing-lists (there are a total of 8). This is represented on the screen with a checkbox for each mailing-list.
I would like to use NHibernate to update these in one go. A very simple sql query would be:
update mail_subscriptions set subscribed = true where mailing_list_id in (21,14,15,19) and user_id = 'me'
What is the cleanest way to perform this update via NHibernate so that I can make a single round trip to the database?
Thanks in advance
JP
NHibernate might not be able to update the mail_subscriptions in the way you have shown above but it can do it in a single round trip to the DB using batched queries.
This example considers Subscriptions mapped as a HasMany using Component although roughly the same technique can be used if the mapping was just a plain HasMany. I am also assuming that each user already has rows in the mail_subscriptions table for each mailing list set to false for subscribed.
public class User{
public virtual string Id {get; set;}
public virtual IList<MailSubscription> Subscriptions {get; set;}
}
public class MailSubscription{
public virtual int ListId {get; set;}
public virtual bool Subscribed {get; set;}
}
public void UpdateSubscriptions(string userid, int[] mailingListIds){
var user = session.Get<User>(userid);
foreach(var sub in
user.Subscriptions.Where(x=> mailingListIds.Contains(x.ListId))){
sub.Subscribed=true;
}
session.Update(user);
}
Now when the unit of work completes you should see SQL like this produced sent as a single round trip to the DB.
update mail_subscriptions set subscribed=true where user_id='me' and listid=21
update mail_subscriptions set subscribed=true where user_id='me' and listid=14
update mail_subscriptions set subscribed=true where user_id='me' and listid=15
update mail_subscriptions set subscribed=true where user_id='me' and listid=19
I think the NHibernate feature you seek is known as Executable DML.
Ayende has a blog post giving an example at http://ayende.com/blog/4037/nhibernate-executable-dml .
Depending on your names of your entities and their properties, and assuming you have an ISession instance variable called session, you would need to execute an HQL query something like:
session.CreateQuery("update MailSubscriptions set Subscribed = true where MailingList.Id in (21,14,15,19) and User.Id = 'me'")
.ExecuteUpdate();
Now, having said that, I think in the use case you describe (updating a handful of entries within a collection on a single aggregate root), there is no need to use Executable DML. Mark Perry has the right idea - you should simply modify the booleans on the appropriate entities and flush the session in the usual way. If ADO.NET batching is configured appropriately, then the child entries will cause multiple update statements to be sent to the RDBMS in a single database call.
I'm looking at using doctrine for an application I'm working on - but after reading the documentation I'm having trouble conceptualizing how to represent the database structure we have in terms of entities.
I have many tables which have partner tables which hold translation data like the following....
Where I would like to have one Entity (Navigation Element) which had access to the 'label' field depending on what Language I set in my application. The following from the Doctrine documentation seems to suggest that you need to define one (single) table which is used to persist an entity
http://www.doctrine-project.org/docs/orm/2.0/en/reference/basic-mapping.html
By default, the entity will be
persisted to a table with the same
name as the class name. In order to
change that, you can use the #Table
annotation as follows:
Or do I need to define two entities and link them (or allow the translation table to inherit from the element table).
And what strategy would I use to always insert a language_id clause to the Join (to ensure I'm pulling the right label for the currently set language). Is this something I would define in the entity itself, or elsewhere?
This seems to suit a One-To-Many Bidirectional association. This is the scenario from that page translated to your situation:
/** #Entity */
class NavigationElement
{
// ...
/**
* #OneToMany(targetEntity="NavigationElementTranslation", mappedBy="navigationElement")
*/
private $translations;
// ...
public function __construct() {
$this->translations = new \Doctrine\Common\Collections\ArrayCollection();
}
}
/** #Entity */
class NavigationElementTranslation
{
// ...
/**
* #ManyToOne(targetEntity="NavigationElement", inversedBy="translations")
* #JoinColumn(name="navigation_element_id", referencedColumnName="id")
*/
private $navigationElement;
// ...
}
You could add a getLabel($languageId) method to the NavigationElement entity that searches through the translations to get the correct label:
public function getLabel($languageId) {
foreach($this->translations as $trans) {
if($trans->languageId == $languageId)
return $trans->label;
}
throw new InvalidArgumentException();
}
And you could use the following DQL to ensure you only load the translation you want into the $translations property:
$query = $em->createQuery(
"SELECT ne, net
FROM Entity\NavigationElement ne
JOIN ne.translations net WITH net.languageId = :langId"
);
$query->setParameter('langId', $languageId);
$navigationElements = $query->execute();
This situation sounds like one where you would want to cache aggressively. Make sure you look into Doctrine 2's caching mechanisms too.
Also, internationalization can be handled reasonably well in PHP with gettext if you find join tables for translations start to become unmanageable.
I would also direct anyone who has to tackle this same problem to take a look at the following doctrine extension.
http://www.gediminasm.org/article/translatable-behavior-extension-for-doctrine-2
Let me explain the problem - hopefully I have defined it well in the title but I want to be sure.
I have a linq query that pulls back a bunch of objects (say Foos). Each Foo holds a reference to a User. Each User holds a reference to a Person:
public class Foo
{
//properties omitted...
public User CreatedBy {get;}
}
public class User
{
//properties omitted...
public Person Person {get;set;}
}
As the object structure would suggest, in the database, Foo relates many-to-one to User, and User relates many-to-one to Person.
When I run the query, I get a single SELECT for the Foos, then a SELECT each for all the Users and People. Clearly I would much prefer a single SELECT with a couple of joins.
I don't necessarily want to specify in my mapping config that Foos ALWAYS eager fetch the User, or that Users ALWAYS eager fetch the Person, but I would like to be able to specify that in this instance.
Is there a way to do that?
Thanks
David
All the NHibernate query methods have ways of specifying eager fetching.
For Criteria, you have SetFetchMode.
For HQL, you have [inner|left] join fetch.
For Linq yo have Expand (2.x contrib) / Fetch (3.x).
For SQL you have AddJoin.
Both Udi Dahan and Ritesh Rao offer example implementations of dynamic fetching strategies for NHibernate, this should give you a good starting point.
Additionally to Diegos nice answer: You can also use batching. This reduces the N+1 problem without much pain:
use batch-size on class level:
<class name="Person" batch-size="20">
...
</class>
use batch-size on collection level:
<map
name="SomeCollection"
batch-size="20">
...
</map>
When ever one of these references is loaded, NHibernate loads 20 at once using a query like this:
select ... from Person where user_fk in (23, 34, 6, 667, 6745, 234 ....)
So it turns N+1 to N / 20 + 1, which is pretty good.
I have 3 related objects (Entry, GamePlay, Prize) and I'm trying to find the best way to query them for what I need using NHibernate. When a request comes in, I need to query the Entries table for a matching entry and, if found, get a) the latest game play along with the first game play that has a prize attached. Prize is a child of GamePlay and each Entry object has a GamePlays property (IList).
Currently, I'm working on a method that pulls the matching Entry and eagerly loads all game plays and associated prizes, but it seems wasteful to load all game plays just to find the latest one and any that contain a prize.
Right now, my query looks like this:
var entry = session.CreateCriteria<Entry>()
.Add(Restrictions.Eq("Phone", phone))
.AddOrder(Order.Desc("Created"))
.SetFetchMode("GamePlays", FetchMode.Join)
.SetMaxResults(1).UniqueResult<Entry>();
Two problems with this:
It loads all game plays up front. With 365 days of data, this could easily balloon to 300k of data per query.
It doesn't eagerly load the Prize child property for each game. Therefore, my code that loops through the GamePlays list looking for a non-null Prize must make a call to load each Prize property I check.
I'm not an nhibernate expert, but I know there has to be a better way to do this. Ideally, I'd like to do the following (pseudocode):
entry = findEntry(phoneNumber)
lastPlay = getLatestGamePlay(Entry)
firstWinningPlay = getFirstWinningGamePlay(Entry)
The end result of course is that I have the entry details, the latest game play, and the first winning game play. The catch is that I want to do this in as few database calls as possible, otherwise I'd just execute 3 separate queries.
The object definitions look like:
public class Entry
{
public Guid Id {get;set;}
public string Phone {get;set;}
public IList<GamePlay> GamePlays {get;set;}
// ... other properties
}
public class GamePlay
{
public Guid Id {get;set;}
public Entry Entry {get;set;}
public Prize Prize {get;set;}
// ... other properties
}
public class Prize
{
public Guid Id {get;set;}
// ... other properties
}
The proper NHibernate mappings are in place, so I just need help figuring out how to set up the criteria query (not looking for HQL, don't use it).
since you are doing this in each request maybe it should be better to set up two formula-properties in your entity.
The first one should fetch the latest Gameplay-Id and the other the first Gameplay-Id with a not Null property
this could be as such in the xml mapping file of Entry
<property name="LatestGameplay" formula="select top(1)gp.Id from Gameplay gp where gp.FK_EntryId = PK_EntryId order by gp.InsertDate desc" />
this leaves you with the Gameplay Id's on the Entry entity and after you fetch it it would require another round trip to the DB to GetById-fetch the gameplay's
Alternatively you could work-around using filters.
Set the collection back to "lazy"
and create these nice filters
Gameplay latest = NHibernateSession.CreateFilter(entry.GamePlays , "order by InsertDate desc").SetMaxResults(1).SetFirstResult(1).UniqueResult<Gameplay>();
Gameplay winner = NHibernateSession.CreateFilter(entry.GamePlays , "where FK_PrizeId is not null order by InsertDate asc ").SetMaxResults(1).SetFirstResult(1).UniqueResult<Gameplay>();
And IFilters can be used in a multiquery as so have 2 db hits: one for the original Entry and one for the multiquery.
Last but not least, you could define 2 bags in the Entry entity, one IList<GamePlay> Latest and one IList<Gameplay> Winner which in the Entry mapping file would be filtered with the appropriate query (although i don't remember now if you can define TOP clauses in the filters) and set those as non-lazy. Then with a single round-trip you can have all the data you want with the following (ugly) syntax
Entry entry = findEntry(phoneNumber);
Gameplay winner = entry.Winner[0]; //check this if null first
Gameplay Latest = entry.Latest[0]; //ditto
note that of all the solutions the 3rd is the one that provides a mechanism to generate additional queries, as the bag can be used in a Criteria/HQL query
I would like to know if there is a way to disable automatic loading of child records in nHibernate ( for one:many relationships ).
We can easily switch off lazy loading on properties but what I want is to disable any kind of automatic loading ( lazy and non lazy both ). I only want to load data via query ( i.e. HQL or Criteria )
I would still like to define the relationship between parent child records in the mapping file to facilitate HQL and be able to join parent child entities, but I do not want the child records to be loaded as part of the parent record unless a query on the parent record
explicitly states that ( via eager fetch, etc ).
Example:
Fetching Department record from the database should not fetch all employee records from the database because it may never be needed.
One option here is to set the Employees collection on Department as lazy load. The problem with this approach is that once the object is given to the calling API it can 'touch' the lazy load property and that will fetch the entire list from the db.
I tried to use 'evict' - to disconnect the object but it does not seem to be working at all times and does not do a deep evict on the object.
Plus it abstracts the lazy loaded property type with a proxy class that plays havoc later in the code where we are trying to operate on the object via reflection and it encounters unexpended type on the object.
I am a beginner to nHibernate, any pointers or help would be of great help.
Given your request, you could simply not map from Department to Employees, nor have an Employees property on your department. This would mean you always have to make a database hit to find the employees of a database.
Aplogies if these code examples don't work out of the box, I'm not near a compiler at the moment
So, your department class might look like:
public class Department
{
public int Id { get; protected set; }
public string Name { get; set; }
/* Equality and GetHashCode here */
}
and your Employee would look like:
public class Employee
{
public int Id { get; protected set; }
public Name Name { get; set; }
public Department Department { get; set; }
/* Equality and GetHashCode here */
}
Any time you wanted to find Employees for a department, you've have to call:
/*...*/
session.CreateCriteria(typeof(Employee))
.Add(Restrictions.Eq("Department", department)
.List<Employee>();
Simply because your spec says "Departments have many Employees", doesn't mean you have to map it as a bi-directional association. If you can keep your associated uni-directional, you can really get your data-access to fly too.
Google "Domain Driven Design" Aggregate, or see Page 125 of Eric Evan's book on Domain Driven Design for more information
You can have the lazy attribute on the collection. In your example, Department has n employees, if lazy is enabled, the employees will not be loaded by default when you load a department : http://www.nhforge.org/doc/nh/en/#collections-lazy
You can have queries that explicitly load department AND employees together. It's the "fetch" option : http://www.nhforge.org/doc/nh/en/#performance-fetching-lazy