With my only ORM knowledge being L2S/EF, I was surprised when the following code inserted a row into the database before I called repo.Save:
var repo = new UserRepository();
var user = new User { Name = "test" }
repo.Add(user);
//repo.Save();
Repo looks like this:
public void Add(T entity)
{
session.Save(entity);
}
public void Save()
{
session.Flush();
}
After some digging, it seems NHibernate needs to make the insert happen right away in order to get the ID of the new entity (since it's using an auto increment ID). But L2S/EF doesn't work like this; I can add many entities and save them all at the end.
Question is: is there a way to achieve the same thing with NHibernate, while still using auto increment IDs, and out of interest does anyone know why it works like this?
Fabio Maulo already blogged about the usage of identity generator a few times. The answer is: use hilo, guid.comb or something like this.
NHibernate needs the identity because every entity in the session (they are called "persistent entities") needs to be identified. The identity is also normally used to determine if the record already exists in the database (unsaved value).
session.Save actually only makes a transient entity persistent. When the database is generating the id, it needs to be stored to get the id. If NH can create the id itself (eg using hilo), it could be stored next time when the session gets flushed.
Related
I just upgraded a project from RavenDB 3.5 to 4.0 and one of the biggest change I noticed is the way they change the way Ids are generated.
In my project most of the collections have a basic id structure like "[collection name]/[progressive id]", where the progressive id is an integer, and not the new default "[progressive]-[node]".
Following the documentation I specified the pattern id for new documents as "[collection name]|" and is actually generating unique/progressive/integer ids.
The problem is when I've to save transactionally 2 or more documents and reference them between themselves. Let's say I've two kind of object:
User entity
{
"Id": "users/1",
...
}
User address entity
{
"Id": "userAddresses/1",
"UserId": "users/1",
...
}
Where in the second document I need to reference the first one via the UserId field.
Before the version 4.0 I was able, in the same transaction, to do something like:
User newUser = new User();
session.Store(newUser)
UserAddress newUserAddress = new UserAddress();
newUserAddress.UserId = newUser.Id;
session.Store(newUserAddress);
session.SaveChanges();
After the session.Store(newUser) if I accessed the newUser.Id property I was able to see the generated Id. Now I just see "users|", I've to wait after the SaveChanges() to see the generated Ids.
This behaviour seems to happen only for Identities Ids, if I use the id structure "[collection name]/[progressive]-[node]" I'm able to see the generated id right after the Store().
Is it by design? Is there a way to force the old behaviour?
OR How can I manage transactionally a situation like this one using progressive/integer ids?
In RavenDB v4.0 you have the same behavior. After you call to session.Store(entity), or await session.StoreAsync(entity) for the async session, you should have the entity.Id filled with the ID.
It is setting the ID using the HiLo approach, which you can read about it here:
https://ravendb.net/docs/article-page/4.0/Csharp/server/kb/document-identifier-generation#hilo-algorithm
The only difference in RavenDB v4.0 that the ID would be like: users/1-A instead of users/1 in the previous versions.
I am trying get a record updated from database with QueryOver.
My code initially creates an entity and saves in database, then the same record is updated on database externally( from other program, manually or the same program running in other machine), and when I call queryOver filtering by the field changed, the query gets the record but without latest changes.
This is my code:
//create the entity and save in database
MyEntity myEntity = CreateDummyEntity();
myEntity.Name = "new_name";
MyService.SaveEntity(myEntity);
// now the entity is updated externally changing the name property with the
// "modified_name" value (for example manually in TOAD, SQL Server,etc..)
//get the entity with QueryOver
var result = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>();
The previous statement gets a collection with only one record(good), BUT with the name property established with the old value instead of "modified_name".
How I can fix this behaviour? First Level cache is disturbing me? The same problem occurs with
CreateCriteria<T>();
The session in my NhibernateHelper is not being closed in any moment due application framework requirements, only are created transactions for each commit associated to a session.Save().
If I open a new session to execute the query evidently I get the latest changes from database, but this approach is not allowed by design requirement.
Also I have checked in the NHibernate SQL output that a select with a WHERE clause is being executed (therefore Nhibernate hits the database) but don´t updates the returned object!!!!
UPDATE
Here's the code in SaveEntity after to call session.Save: A call to Commit method is done
public virtual void Commit()
{
try
{
this.session.Flush();
this.transaction.Commit();
}
catch
{
this.transaction.Rollback();
throw;
}
finally
{
this.transaction = this.session.BeginTransaction();
}
}
The SQL generated by NHibernate for SaveEntity:
NHibernate: INSERT INTO MYCOMPANY.MYENTITY (NAME) VALUES (:p0);:p0 = 'new_name'.
The SQL generated by NHibernate for QueryOver:
NHibernate: SELECT this_.NAME as NAME26_0_
FROM MYCOMPANY.MYENTITY this_
WHERE this_.NAME = :p0;:p0 = 'modified_name' [Type: String (0)].
Queries has been modified due to company confidential policies.
Help very appreciated.
As far as I know, you have several options :
have your Session as a IStatelessSession, by calling sessionFactory.OpenStatelesSession() instead of sessionFactory.OpenSession()
perform Session.Evict(myEntity) after persisting an entity in DB
perform Session.Clear() before your QueryOver
set the CacheMode of your Session to Ignore, Put or Refresh before your QueryOver (never tested that)
I guess the choice will depend on the usage you have of your long running sessions ( which, IMHO, seem to bring more problems than solutions )
Calling session.Save(myEntity) does not cause the changes to be persisted to the DB immediately*. These changes are persisted when session.Flush() is called either by the framework itself or by yourself. More information about flushing and when it is invoked can be found on this question and the nhibernate documentation about flushing.
Also performing a query will not cause the first level cache to be hit. This is because the first level cache only works with Get and Load, i.e. session.Get<MyEntity>(1) would hit the first level cache if MyEntity with an id of 1 had already been previously loaded, whereas session.QueryOver<MyEntity>().Where(x => x.id == 1) would not.
Further information about NHibernate's caching functionality can be found in this post by Ayende Rahien.
In summary you have two options:
Use a transaction within the SaveEntity method, i.e.
using (var transaction = Helper.Session.BeginTransaction())
{
Helper.Session.Save(myEntity);
transaction.Commit();
}
Call session.Flush() within the SaveEntity method, i.e.
Helper.Session.Save(myEntity);
Helper.Session.Flush();
The first option is the best in pretty much all scenarios.
*The only exception I know to this rule is when using Identity as the id generator type.
try changing your last query to:
var result = NhibernateHelper.Session
.QueryOver<MyEntity>()
.CacheMode(CacheMode.Refresh)
.Where(param => param.Name == "modified_name")
if that still doesn't work, try add this after the query:
NhibernateHelper.Session.Refresh(result);
After search and search and think and think.... I´ve found the solution.
The fix: It consist in open a new session, call QueryOver<T>() in this session and the data is succesfully refreshed. If you get child collections not initialized you can call HibernateUtil.Initialize(entity) or sets lazy="false" in your mappings. Take special care about lazy="false" in large collections, because you can get a poor performance. To fix this problem(performance problem loading large collections), set lazy="true" in your collection mappings and call the mentioned method HibernateUtil.Initialize(entity) of the affected collection to get child records from database; for example, you can get all records from a table, and if you need access to all child records of a specific entity, call HibernateUtil.Initialize(collection) only for the interested objects.
Note: as #martin ernst says, the update problem can be a bug in hibernate and my solution is only a temporal fix, and must be solved in hibernate.
People here do not want to call Session.Clear() since it is too strong.
On the other hand, Session.Evict() may seem un-applicable when the objects are not known beforehand.
Actually it is still usable.
You need to first retrieve the cached objects using the query, then call Evict() on them. And then again retrieve fresh objects calling the same query again.
This approach is slightly inefficient in case the object was not cached to begin with - since then there would be actually two "fresh" queries - but there seems to be not much to do about that shortcoming...
By the way, Evict() accepts null argument too without exceptions - this is useful in case the queried object is actually not present in the DB.
var cachedObjects = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>();
foreach (var obj in cachedObjects)
NhibernateHelper.Session.Evict(obj);
var freshObjects = NhibernateHelper.Session
.QueryOver<MyEntity>()
.Where(param => param.Name == "modified_name")
.List<T>()
I'm getting something very similar, and have tried debugging NHibernate.
In my scenario, the session creates an object with a couple children in a related collection (cascade:all), and then calls ISession.Flush().
The records are written into the DB, and the session needs to continue without closing. Meanwhile, another two child records are written into the DB and committed.
Once the original session then attempts to re-load the graph using QueryOver with JoinAlias, the SQL statement generated looks perfectly fine, and the rows are being returned correctly, however the collection that should receive these new children is found to have already been initialized within the session (as it should be), and based on that NH decides for some reason to completely ignore the respective rows.
I think NH makes an invalid assumption here that if the collection is already marked "Initialized" it does not need to be re-loaded from the query.
It would be great if someone more familiar with NHibernate internals could chime in on this.
After inserting a particular type of entity I want to make sure that it is always evicted from the Session.
It seems I can do this with IPostInsertEventListener, but I'm not sure if it is safe.
Will this work:
public class LogUpdatesPostInsertEventListener : IPostInsertEventListener
{
public void OnPostInsert(PostInsertEvent insertEvent)
{
var entity = insertEvent.Entity;
if (entity is Foo)
{
insertEvent.Session.Evict(entity);
}
}
}
I think given what you want to achieve the code that you have looks correct. I am wondering if you should add an explicit Session.Flush() to make sure that the insert is committed to the database, but it depends on your requirement.
Also, make sure that you are working with only one session during one single unit of work for example for one HTTP request.
Ok, each and every time I get into this situation, I struggle back and forth until I find a way to solve it (and that is usually not the way I would have liked to solve it).
What I'm talking about is disconnected entities in EF that should update existing entities in the database.
I'll give an example of my problem here (this example is the last time I got into this problem that caused me to write this question).
I have a WCF service that uses Entity Framework as well. The other program that have added a service reference to my service have gotten proxy versions of the Entities as normal.
The case is that the consumer of the service now construct a object of this proxy class, and call the method UpdateEntity on the WCF service. This entity has a foreign key to another type of entities, and the primary key of the entity I want to link this new entity to is also sent as a parameter to this method. In this case, I want the entity with the same primary key in the database to be updated. It seems simple enough right?
My method looks something like this now:
public bool ChangeEntity(MyEntity entity, int otherTableForignKey)
{
//first I verify that the entity to update exist in the system
var entitytochange = entityContext.MyEntities.FirstOrDefault(e => e.Name == entity.Name);
if (systemtochange == null) return false;
try
{
entity.ForignEntity = entityContext.ForeignEntities.FirstOrDefault(f => f.Key == otherTableForignKey);
//code for updating the entity should go here, but I'm nor sure what
entityContext.SaveChanges();
return true;
}
catch (Exception exc)
{
return false;
}
}
I tried many different combinations of ApplyCurrentValues, Attach, setting ObjectState to Modified and so on, but I get either the error message that I can't add a new entity with the same key as an existing entity, that the object state of the new object can't be Added and so on.
So my question is: What is the best way to do this without writing code that looks like a big hack.
The only way I got this working now was to just set the properties of entitytochange manually with the properties of entity, but it is a bad solution since any added properties to MyEntity will break the code if I don't remember to add code in this method as well, and it seems there really should be another way that is better.
EDIT
When I put entityContext.MyEntities.ApplyCurrentValues(entity); where my comment is put above, I get the following exception on this line:
The existing object in the ObjectContext is in the Added state. Changes can only be applied when the existing object is in an unchanged or modified state.
However, if I remove this line above entity.ForignEntity = entityContext.ForeignEntities.FirstOrDefault(f => f.Key == otherTableForignKey); then the ApplyCurrentValues works without any problems.
Why would me setting the ForeignEntity of the object set it to Added state? So it seems that setting a Property on the Detached entity, attaches it to the context with a state of added?
I need to write a row to the database regardless of whether it already exists or not. Before using NHibernate this was done with a stored procedure. The procedure would attempt an update and if no rows were modified it would fallback to an insert. This worked well because the application doesn't care if the record exists.
With NHibernate, the solutions I have found require loading the entity and modifying it, or deleting the entity so the new one can be inserted. The application does have to care if the record already exists. Is there a way around that?
Does the Id Matter?
Assigned Id
The object has a keyword as an assigned id and is the primary key in the table.
I understand that SaveOrUpdate() will call the Save() or Update() method as appropriate based on the Id. Using an assigned id, this won't work because the id isn't an unsaved-value. However a Version or Timestamp field could be used as an indicator instead. In reality, this isn't relevant because this only reflects on whether the object in memory has been associated with a record in the database; it does not indicate if the record exists or not in the database.
Generated Id
If the assigned id were truly the cause of the problem, I could use a generated id instead of the keyword as the primary key. This would avoid the NHibernate Insert/Update issue as it would effectively always insert. However, I still need to prevent duplicate keywords. With a unique index on the keyword column it will still throw an exception for a duplicate keyword even if the primary key is different.
Another Approach?
Perhaps the problem isn't really with NHibernate, but the way this is modeled. Unlike other areas of the application, this is more data-centric rather object-centric. It is nice that NHibernate makes it easy to read/write and eliminates the stored procedures. But the desire to simply write without regard to existing values doesn't fit well with the model of an object's identity model. Is there a better way to approach this?
I`m using
public IList<T> GetByExample<T>(T exampleInstance)
{
return _session.CreateCriteria(typeof(T))
.Add(Example.Create(exampleInstance))
.List<T>();
}
public void InsertOrUpdate<T>(T target)
{
ITransaction transaction = _session.BeginTransaction();
try
{
var res=GetByExample<T>(target);
if( res!=null && res.Count>0 )
_session.SaveOrUpdate(target);
else
_session.Save(target);
transaction.Commit();
}
catch (Exception)
{
transaction.Rollback();
throw;
}
finally
{
transaction.Dispose();
}
}
but FindByExample method returns all objects alike not objects with the exact ID what do you suggest ? since I have only object as parameter I don't have access to its specific ID field so I cannot use session.get(Object.class(), id);
Typically, NHibernate can rely on the unsaved-value to determine whether it should insert or create the entity. However, since you are assigning the ID, to NHibernate it looks like your entity has already been persisted. Therefore, you need to rely on versioning your object to let NHibernate know that it is a new object. See the following link for how to version your entity:
http://web.archive.org/web/20090831032934/http://devlicio.us/blogs/mike_nichols/archive/2008/07/29/when-flushing-goes-bad-assigned-ids-in-nhibernate.aspx
Use the session.SaveOrUpdate(object) method.
You can do
Obj j = session.get(Object.class(), id);
if (j != null)
session.merge(myObj);
else
session.saveOrUpdate(myObj);
Query objects where keyword = x, take FirstOrDefault. If it's null, Add new object, if it exists, update object that you got and call saveOrUpdate on it.
This worked for me:
Implementation
public void InsertOrUpdate<TEntity, TId>(TEntity entity) where TEntity : IIdentificableNh<TId>
{
var anyy = session.Get<TEntity>(entity.Id);
if (anyy != null)
{
session.Evict(anyy); //dispatch all data loaded, to allow updating 'entity' object.
session.Update(entity);
}
else
{
session.Save(entity);
}
session.Flush();
}
Entity
public class Caracteristica : IIdentificableNh<int>
{
public virtual int Id { get; set; }
public virtual string Descripcion { get; set; }
}
I had to create an interface (IIdentificableNh) that allows me to access the Id property value.
Usage example:
session.InsertOrUpdate<Caracteristica, int>(new Caracteristica { Id = 2, Descripcion = "Caracteristica2" });
call hibernate.saveOrUpdate() which will check if the object is in the database, update it if it is, and save (i.e. insert) it if it is not.