I don't like the idea of proxy and lazy loading. I don't need that. I want pure POCO. And I want to control loading associations explicitly when I need.
Here is entity
public class Post
{
public long Id { get; set; }
public long OwnerId { get; set; }
public string Content { get; set; }
public User Owner { get; set; }
}
and mapping
<class name="Post">
<id name="Id" />
<property name="OwnerId" />
<property name="Content" />
<many-to-one name="Owner" column="OwnerId" />
</class>
However if I specify lazy="false" in the mapping, Owner is always eagerly fetched.
I can't remove many-to-one mapping because that also disables explicit loading or a query like
from x in session.Query<Post>()
where x.Owner.Title == "hello"
select x;
I specified lazy="true" and set use_proxy_validator property to false. But that also eager loads Owner.
Is there any way to load only Post entity?
In short, it is not possible with out of box NH. But here is attempt at just, lazy loading without proxies
http://thinkbeforecoding.com/post/2009/02/07/Lazy-load-and-persistence-ignorance
Set the class User to lazy = false on the mapping
<class name="User" table="Users" lazy="false">
Remove this property <property name="OwnerId" />... to get the owner id you can use Owner.Id. This will not trigger a lazy load. Owner will only be loaded if you hit any property besides the id. To make it a flat/simple POCO, you can use projections and ResultTransformers.
Davy Brion - Must Everything be Virtual with NHibernate
Related
I have two objects, Case and Note. A Case can have gobs of Notes, like, in the thousands. We are trying to load them asynchronously, in batches, and stream them to the UI so there is no delay waiting for them all to load.
The class/mappings are
public class Case
{
public virtual IList<Note> Notes { get; protected set; }
}
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="SCMS.TAMS.BusinessEntities" namespace="SCMS.TAMS.BusinessEntities">
<class name="Case" table="Cases">
<bag name="Notes" inverse="true" cascade="all" lazy="true">
<key column="CaseID" />
<one-to-many class="Note" />
</bag>
</class>
</hibernate-mapping>
public class Note
{
public virtual Case Case {get; set;}
public virtual long CaseId {get; set;}
}
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="SCMS.TAMS.BusinessEntities" namespace="SCMS.TAMS.BusinessEntities" default-lazy="true">
<class name="Note" table="CaseNotes">
<many-to-one name="Case" column="CaseID"/>
<property name="CaseId" column="CaseID" />
</class>
</hibernate-mapping>
Now, when I call
NHibernateSession.Query<Note>().Where(n => n.CaseId == 123).Skip(0).Take(10).ToList();
to load the first 10 Notes for Case 123, the thing loads the Case object, which takes about 30 seconds because there's lots of other things on it, and other logic when it gets loaded, etc., none of which I need/want at this time. All I want/need are the 10 Notes.
I've tried all sorts of variations on this mapping and none of them have worked. What am I doing wrong here?
How are you using this query? is it some thing for the UI? liking showing in a grid or something? or are you performing business logic in a component?
Either way you want to project into another object. Your query right now returns a list of notes which is then going to load that parent object per the mappings.
So if you are using this query to send the information to the UI of an asp.net mvc application, project directly into your view model
NHibernateSession.Query<Note>().Where(n => n.CaseId == 123).Select(n => new SomeViewModel { Prop1 = n.Prop1, Prop2 = n.Prop2 ...}).Skip(0).Take(10).ToList();
or create an anonymous object
NHibernateSession.Query<Note>().Where(n => n.CaseId == 123).Select n => new { n.Prop1, n.Prop2, ...}).Skip(0).Take(10).ToList();
This will keep the parent object from loading. It also has the added benefit that you are only querying the information you need because the query be limited to the data you are projecting.
Important to know is that if all above is true...
this is the real mapping (which is not it is just an obvious extract)
<class name="Note" table="CaseNotes">
<many-to-one name="Case" column="CaseID"/>
...
this is the class (again extract without ID)
public class Note
{
public virtual Case Case {get; set;}
public virtual long CaseId {get; set;}
}
and that would be a UNIT TEST statement to load notes:
var list = NHibernateSession
.Query<Note>()
.Where(n => n.CaseId == 123)
.Skip(0).Take(10)
.ToList();
then NHibernate will NEVER load the Case object. Never. Because:
NHibernate is lazy, just live with it
The reason, the trigger to load related reference (Case property) must be something explicit.
Mostly:
there is usage of the Case object somewhere. E.g. in override of the GetHashCode() the Case.ID is used
Or:
there is a serialization or DTO conversion which does touch the Case property
In those case, NHibernate must load that...
So, create few unit tests with basic queries and assure that your the is really as shown above. Then it will work as expected
i have old legacy DB which has dead links in their tables. I have class mapped in nhibernate like this:
<class name="Visible" table="table_visible">
<composite-id>
<key-many-to-one column="object_id" name="ObjectA" />
<key-many-to-one column="sub_object_id" name="SubObject" />
</composite-id>
<property column="visible" name="VisibleRow" />
</class>
and:
public class Visible
{
public virtual ObjectAClass ObjectA { get; set; }
public virtual SubObjectClass SubObject { get; set; }
public virtual bool VisibleRow { get; set; }
public override bool Equals(object obj)
{
var other = ((Visible)obj);
return this.ObjectA.Equals(other.ObjectA) && this.SubObject.Equals(other.SubObject);
}
public override int GetHashCode()
{
return this.ObjectA.GetHashCode() + (this.SubObject != null? this.SubObject.GetHashCode(): 0);
}
}
Now all works fine when all joins in database are correct, but when i find such sub_object_id which doesnt have entity, nhibernate throws me error
No row with the given identifier exists:[SubObject#123]
Is there a way to map composite key so that when its subentity is not found, the whole entity wouldnt be loaded (like with inner join)?
NHibernate v2.0.50727
Following Daniel Schilling idea of fetch Visible entities with a where exists sub-query, found that there is loader element available in mappings.
<class name="ObjectA" table="table_object">
.........
<set name="VisibleList" cascade="all" lazy="false" inverse="true">
<key column="object_id" />
<one-to-many class="Visible" />
<loader query-ref="valid_entities"/>
</set>
</class>
<sql-query name="valid_entities">
<load-collection alias="v" role="ObjectA.VisibleList"/>
SELECT {v.*}
FROM table_visible v
INNER JOIN table_sub_entities e ON e.sub_entity_id=v.sub_entity_id
WHERE v.object_id=?
</sql-query>
And nothing else needed to be changed.
<key-many-to-one column="sub_object_id" name="SubObject" not-found="ignore" />
... may be helpful. From the NHibernate Documentation...
ignore will treat a missing row as a null association
Please be aware of the performance penalty associated with using this option. Whenever NHibernate fetches a Visible entity, it will also have to fetch SubObject. If you don't go ahead and fetch it in your query, this means that NHibernate will be issuing lots of lazy loads.
This doesn't meet your "when its sub-entity is not found, the whole entity wouldn't be loaded" goal. Instead NHibernate would give you an entity with a null sub-entity. If you want that inner-join-like behavior, then I think you would need to fetch your Visible entities with a where exists sub-query to make sure the SubObject actually exists.
The best option would be to fix the data in the database and add a foreign key constraint.
I just ran across this: Relations with not-found="ignore". I promise I'm not copying Ricci's content - I'm writing this from my own experience.
I'm using NHibernate 2.2 for my database work and I've faced an issue recently. I have a class called PrescDrugItem which is shown below
public class PrescDrugItem
{
public virtual int ItemNumber { get; set; }
[DataMember]
public virtual int AmountIssued { get; set; }
[DataMember]
public virtual string TimePeriod { get; set; }
}
following is the mapping file
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly ="DataContractsLib"
namespace="DataContractsLib.Prescription" >
<class name="PrescDrugItem">
<id name="ItemNumber" type="Int32">
<generator class="native" />
</id>
<property name="AmountIssued" type="Int32" />
<property name="TimePeriod" type="String" length="30" />
</class>
my problem is, now I need to add another property to the class Item (say ItemTradeName etc), but I dont want it to be saved to the database( because I want to use this new property to store some data temporary). I tried update=false and insert=false in the mapping file but no success yet. Could you guys please tell me is this possible thing to do . Thank you.
If it's not to be fetched from the database either, just add it as a normal property of your class and don't map it.
I have an odd problem in my current project. Lazy loading for queries does not work. When I query a list, nhibernate fetches all associations separately.
I extracted small parts of it and put it into a separate solution. Basically what I've got now, is a Account-Table and a AccountSync-Table. Both have an ID and a URL, while the ID is just a db-guid.
My classes are:
public class HippoAccount
{
public virtual Guid Id { get; set; }
public virtual string Url { get; set; }
public virtual HippoAccountSync Sync { get; set; }
}
public class HippoAccountSync
{
public virtual Guid Id { get; set; }
public virtual string Url { get; set; }
public virtual HippoAccount Account { get; set; }
}
When I now load a object via it's guid:
var account = session.Load<HippoAccount>(accountId);
Console.WriteLine(NHibernateUtil.IsPropertyInitialized(account, "Sync"))
... it returns false and account itself is a proxy.
But when loading a list via the criteria API:
var account = (HippoAccount)session
.CreateCriteria(typeof (HippoAccount))
.Add(Restrictions.Eq("Id", accountId))
.List()[0];
... the property Sync gets initialized (firing a second select query), and the returned object is not a proxy.
Is that default behaviour? What am I getting wrong?
The mapping is:
<class name="HippoAccount" table="AllAccounts">
<id name="Id" type="guid">
<generator class="guid"/>
</id>
<property name="Url" />
<many-to-one
class="HippoAccountSync"
name="Sync"
not-found="ignore"
property-ref="Url">
<column name="url" />
</many-to-one>
</class>
<class name="HippoAccountSync"
mutable="false"
table="Accounts">
<id name="Id" type="guid">
<generator class="guid"/>
</id>
<property name="Url">
<column name="serviceUri" />
</property>
<many-to-one class="HippoAccount"
name="Account"
property-ref="Url"
not-found="ignore">
<column name="serviceUri" />
</many-to-one>
</class>
After quite some more research, I found the answers. Answers, because there are many things that can prevent lazy loading in NHibernate.
Query vs. session.Load: When fetching an item via session.Load() you get a proxy. But as soon as you access any property, lets say the Url, the object is fetched including all it's associations that doesn't support lazy loading.
property-ref: Lazy loading only works over a objects id. When an property-association is resolved via a different column in the target entity, NH fetches it eagerly. Not that this wouldn't be possible, it's just not implemented: Bug
not-found="ignore" allows invalid foreign keys, that is, if the referenced entity isn't found NH will init the property with null. NH doesn't intercept the property-access for lazy loading, but instead assignes a object proxy. With not-found="ignore" it can't decide if the property should be set to null or a proxy for the given, possibly invalid, foreign key. This could possibly be solved by intercepting the property access.
When disabling not-found="ignore" and property-ref the schema export would generate constraints that enforce a circular reference. Not good! The correct mapping would then be a constrained one-to-one relationship, where the key for HippoAccountSync must have a generator foreign.
Resources
Select statement issued for each not-found=ignore
Lazy-load conflicts with Property-ref in Many-to-One Mapping
Google groups discussion
I have a legacy database that I am mapping using NHibernate. The objects of concern are an Account and a list of Notification objects. The objects look like:
public class Notification
{
public virtual int Id { get; set; }
public virtual DateTime BatchDate { get; set; }
/* other properties */
public virtual Account Account { get; set; }
}
public class Account
{
public virtual int Id { get; set; }
public virtual string AccountNumber { get; set; }
/* other properties */
}
The mapping files look like:
<class name="Account" table="Account" dynamic-update="true">
<id name="Id" column="AccountID">
<generator class="native" />
</id>
<property name="AccountNumber" length="15" not-null="true" />
<!-- other properties -->
</class>
<class name="Notification" table="Notification">
<id name="Id" column="Id">
<generator class="native" />
</id>
<!-- other properties -->
<many-to-one name="Account" class="Account" property-ref="AccountNumber" lazy="proxy">
<column name="AcctNum" />
</many-to-one>
However, when I create a criteria such as
return session.CreateCriteria(typeof(Notification)).List<Notification>();
I am getting a Select N+1 case where each account is loaded even though the Account is never referenced. Why are all of the accounts getting loaded when the many-to-one is mapped as a lazy proxy?
The issue is caused by the property-ref attribute. Lazy loading only works when the many-to-one reference is using the other object's primary key since NHibernate assumes there's a foreign key constraint enforcing the validity of such a value. With a non-primary key (indicated by the property-ref), NHibernate does not make this assumption and thus does not assume the related object must exist. Since it does not want to create a proxy for an object that does not exist (i.e. should be null instead of a proxy), it eagerly fetches the remote object. This same issue exists when not-found="ignore" is specified since this indicates that the foreign key relationship is not enforced and may result in a null reference.
See also:
NHibernate creates proxy via session.Load(), but not via Linq or Criteria API
http://frankmao.com/2007/12/05/lazy-load-conflicts-with-property-ref-in-many-to-one-mapping/