I have an application which performs the following steps:
Places object in session:
def product = Product.get(1)
session["product"] = product
Performs and Ajax call to update a 1-m relationship, and then renders a partial template which displays the new benefits. These should not yet be saved as the user may change their mind, so discard is called:
def product = session["product"]
if ( !product.isAttached() ) {
product.attach()
}
product.addToBenefits( new Benefit( title: "xx" ) )
product.discard()
session["product"] = product
Attempts to save the object in a save action.
def product = session["product"]
if ( !product.isAttached() ) {
product.attach()
}
product.save()
At this point we get the following exception:
org.springframework.orm.hibernate3.HibernateSystemException: reassociated object has dirty collection; nested exception is org.hibernate.HibernateException: reassociated object has dirty collection
Is there anyway to stop this happening, so that I can re-Attach the object, and save it, thus persisting the changes to the products benefits collection?
Don't store the object in the session, store the id, and reload it instead. You're incurring that cost anyway with attach, so you're not saving anything, and causing this problem, plus wasting server memory, which will affect scalability.
Be sure the instance doesn't have unsaved state before calling attach. If you have changed a dettached entity instance and left it in unsaved state, calling attach will throw HibernateSystemException: reassociated object has dirty collection because to reassociate a transient instance with a session Grails uses Session.LockRequest.lock in the attach call chain:
session.buildLockRequest(new LockOptions(lockMode)).lock(entity);
To reassociate a dettached instace with unsaved state call Session.update, Session.merge or Session.saveOrUpdate:
if ( !product.attached ) {
product.withSession { session -> session.saveOrUpdate(product) }
}
Still, storing only the id and reloading is the best option in OP's case as already mentioned. Most of the times that will be the best choice. For instance, you can use load or get:
def product = Product.load(session["productId"])
Related
I have three entities generated by Entity Framework. One is event and this contains navigation properties called frogs and user_bookings. I posted a related question before about performing a sub-query which seems to work but it prevents me overriding lazy loading of a property.
var evts = from evt in context.events.Include("frogs")
where evt.event_id < 10
select evt;
This works - the navigation property frogs gets loaded.
However, when I alter the LINQ to this:
var evts = from evt in context.events.Include("frogs")
where evt.event_id < 10
select new
{
Event = evt,
HasBooked = evt.user_bookings.Any(x => x.user_id == 1)
};
I get an error trying to access the frogs because the ObjectContext no longer exists. I tried removing virtual from the class definition for the event class, but this just results in an empty list of frogs, when they are definitely there!
This is by design. Include is ignored when the query result is a projection, even when the projection contains an entity that could contain the Included properties.
I don't know why EF implemented it this way. If the projection doesn't contain any entities, but is just some type (anonymous or not), there is no Include target, so ignoring it makes sense. But if the projection does contain an Include target (Event in your case) it seems to me that they could have decided to make that work. But, well, they didn't.
Maybe it's because the rules when Include actually has an effect are less obvious than you might expect. In your case, the shape of the query changes after the Include, so it's ignored.
You could work around this by also querying the frogs:
from evt in context.events.Include("frogs")
where evt.event_id < 10
select new
{
Event = evt,
Frogs = evt.frogs,
HasBooked = evt.user_bookings.Any(x => x.user_id == 1)
};
Now each Event will also have its frogs collection filled (because of relationship fixup). But there are two gotchas. The collections are not marked as loaded, so -
if lazy load can occur afterwards, it will occur, making the initial load useless.
if lazy loading can't occur any more (because the context is disposed) you will get an exception.
This means that to make this work you have to disable lazy loading.
I have been struggling for hours with this: I just can't figure a proper way to cache an object queryset result (object = queryset.get()) in order to avoid re-hitting the database on each view request.
This is my current (simplified) code, and as you can see, I override get_object() to add some extra data (not only the today variable), check if object is in sessions and add object to session.
views.py
from myapp import MyModel
from django.core.cache.utils import make_template_fragment_key
from django.views.generic import DetailView
class myClassView(DetailView):
model = MyModel
def get_object(self,queryset=None):
if queryset is None:
queryset = self.get_queryset()
pk = self.kwargs.get(self.pk_url_kwarg, None)
if pk is not None:
queryset = queryset.filter(pk=pk)
else:
raise AttributeError("My error message.")
try:
today = datetime.today().strftime('%Y%m%d')
cache_key = make_template_fragment_key('some_name', [pk, today])
if cache.has_key(cache_key):
object = self.request.session[cache_key]
return object
else:
object = queryset.get()
object.id = my_id
object.today = today
# Add object to session
self.request.session[cache_key] = object
except queryset.model.DoesNotExist:
raise Http404("Error 404")
return object
The above only works if I add the following:
settings.py
SESSION_SERIALIZER = 'django.contrib.sessions.serializers.PickleSerializer'
But I don't like this hack since it is not secure for Django 1.6 and newer versions because, according to How To Use Sessions (Django 1.7 documents):
If the SECRET_KEY is not kept secret and you are using the PickleSerializer, this can lead to arbitrary remote code execution
If I don't add the SESSIONS_SERIALIZER line I get a "django object is not JSON serializable" error. However, elsewhere my code breaks and I get KeyError errors when trying to pull data from session. This issue is solved converting my string keys into integers. Before changing the settings file Django was converting the str keys into integers automatically when session data was getting requested.
So considering this session serializer security issue I'd prefer other option. So I read here and here about caching get_object(), but I just don't get how to fit that into my get_object() bit. I tried..
if cache.has_key(cache_key):
self._object = super(myClassView,self).get_object(queryset=None)
return self._object
...but it fails. This seems the best solution so far. But how do I implement this into my code? Or, is there a better idea? I'm all ears. Thanks!
You should step back and reassess the situation. What are you trying to achieve?
The get_object is a method that get called in the detailed view to access one specific object from the database.
If you access this method the first time the object gets invalidated and cached in the Queryset.
In order to cache the get_queryset method you need a good cache backend like Redis or Memcached in place so that you can do a simple Write-through Cache operation:
if cache.has_key(cache_key):
object = cache.get(cache_key)
return object
else:
object = queryset.get(pk=pk)
cache.set(cache_key,object)
return object
Note that the django objects are serialized in the cache backend and retrieved as objects when deserialised.
That approach is the just a starting point. You cache the object the first time it misses.
You can also add a post_save,post_update signal to save the object in the cache every time the model is saved or updated:
#receiver(post_save, sender=MyModel)
#receiver(post_delete, sender=MyModel)
def add_MyModel_to_cache(sender, **kwargs):
object = kwargs['instance']
cache.set(cache_key,object)
You have to carefully review what you want to cache and when as it is very easy to misjudge requests
Why isn't the exception triggered? Linq's "Any()" is not considering the new entries?
MyContext db = new MyContext();
foreach (string email in {"asdf#gmail.com", "asdf#gmail.com"})
{
Person person = new Person();
person.Email = email;
if (db.Persons.Any(p => p.Email.Equals(email))
{
throw new Exception("Email already used!");
}
db.Persons.Add(person);
}
db.SaveChanges()
Shouldn't the exception be triggered on the second iteration?
The previous code is adapted for the question, but the real scenario is the following:
I receive an excel of persons and I iterate over it adding every row as a person to db.Persons, checking their emails aren't already used in the db. The problem is when there are repeated emails in the worksheet itself (two rows with the same email)
Yes - queries (by design) are only computed against the data source. If you want to query in-memory items you can also query the Local store:
if (db.Persons.Any(p => p.Email.Equals(email) ||
db.Persons.Local.Any(p => p.Email.Equals(email) )
However - since YOU are in control of what's added to the store wouldn't it make sense to check for duplicates in your code instead of in EF? Or is this just a contrived example?
Also, throwing an exception for an already existing item seems like a poor design as well - exceptions can be expensive, and if the client does not know to catch them (and in this case compare the message of the exception) they can cause the entire program to terminate unexpectedly.
A call to db.Persons will always trigger a database query, but those new Persons are not yet persisted to the database.
I imagine if you look at the data in debug, you'll see that the new person isn't there on the second iteration. If you were to set MyContext db = new MyContext() again, it would be, but you wouldn't do that in a real situation.
What is the actual use case you need to solve? This example doesn't seem like it would happen in a real situation.
If you're comparing against the db, your code should work. If you need to prevent dups being entered, it should happen elsewhere - on the client or checking the C# collection before you start writing it to the db.
I do the following:
def currentUser = springSecurityService.currentUser
currentUser.name = "test"
currentUser.save(flush: true)
// some other code
currentUser.gender = "male"
currentUser.save(flush: true) // Exception occurs
This is the exception I get:
ERROR events.PatchedDefaultFlushEventListener - Could not synchronize database state with session
org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)
How can I prevent this error? What is the best solution for that?
I found different approaches:
here that you can use discard()
here that you can use merge()
Which one should I use?
You should use merge - it will update the object to match the current state in the database. If you use discard it will reset the object back to what the database has, discarding any changes. Everything else in the hibernate session you need to manage yourself.
More importantly code should be written in a service so that there is a database transaction, and you should use
save(flush:true)
once only at the end.
def currentUser = springSecurityService.currentUser
currentUser.name = "test"
// currentUser.save(flush: true) // removing this line because if a rollback occurs, then changes before this would be persisted.
// some other code
currentUser.gender = "male"
currentUser.merge() // This will merge persistent object with current state
currentUser.save(flush: true)
I am using the Exchange Web Services Managed API to work with Tasks (Exchange 2007 SP1). I can create them fine. However, when I try to do updates, it works for all of the fields except for the .Body field. Whenever I try to access (read/update) that field, it gives the following error:
"You must load or assign this property before you can read its value."
The code I am using looks like this:
//impersonate the person whose tasks you want to read
Me.Impersonate(userName); //home-made function to handle impersonation
//build the search filter
Exchange.SearchFilter.SearchFilterCollection filter = New Exchange.SearchFilter.SearchFilterCollection();
filter.Add(New Exchange.SearchFilter.IsEqualTo(Exchange.TaskSchema.Categories, "Sales"));
//do the search
EWS.Task exTask = esb.FindItems(Exchange.WellKnownFolderName.Tasks, filter, New Exchange.ItemView(Integer.MaxValue));
exTask.Subject = txtSubject.Text; //this works fine
exTask.Body = txtBody.Text; //This one gives the error implying that the object isn't loaded
The strange thing is that, inspecting the property bag shows that the object contains 33 properties, but {Body} is not one of them. That property seems to be inherited from the base class .Item, or something.
So, do I need to re-load the object as type Item? Or reload it via .Bind or something? Keep in mind that I need to do this with thousands of items, so efficiency does matter to me.
Calling the Load method solved my problem :)
foreach (Item item in findResults.Items)
{
item.Load();
string subject = item.Subject;
string mailMessage = item.Body;
}
I had the same problem when using the EWS. My Code is requesting the events(Appointments) from the
Outlook calendar, at the end I couldn't reach to the body of the Event itself.
The missing point in my situation was the following "forgive me if there is any typo errors":
After gathering the Appointments, which are also derived from EWS Item Class, I did the following:
1- Create a List with the type Item:
List<Item> items = new List<Item>();
2- Added all appointments to items list:
if(oAppointmentList.Items.Count > 0) // Prevent the exception
{
foreach( Appointment app in oAppointmentList)
{
items.Add(app);
}
}
3- Used the exchanged service "I have already created and used":
oExchangeService.LoadPropertiesForItems(items, PropertySet.FirstClassProperties);
now if you try to use app.Body.Text, it will return it successfully.
Enjoy Coding and Best Luck
I forgot to mention the resource:
http://social.technet.microsoft.com/Forums/en-US/exchangesvrdevelopment/thread/ce1e0527-e2db-490d-817e-83f586fb1b44
He mentioned the use of Linq to save the intermediate step, it will help you avoid using the List items and save some memory!
RockmanX
You can load properties using a custom property set. Some properties are Extended properties instead of FirstClassProperties.
Little example:
_customPropertySet = new PropertySet(BasePropertySet.FirstClassProperties, AppointmentSchema.MyResponseType, AppointmentSchema.IsMeeting, AppointmentSchema.ICalUid);
_customPropertySet.RequestedBodyType = BodyType.Text;
appointment.Load(_customPropertySet);