During the life of my ObjectContext, I receive a message saying telling me that a new entity has been added to the data source by some other process (i.e. not tracked by my ObjectContext). How do I add this entity to my local ObjectContext without recreating it?
I've looked at ObjectContext.Refresh() but I'm not sure if this is the right way forwards.
If the entity is available in your code, use the Attach method.
In case this object is not available in your code, the solution is indeed to call the Refresh method with StoreWins RefreshMode for the collection the object was added into.
I recommend you to take a look at the Self-Tracking Entities as well.
Is the answer not in the question title - long running object contexts are not a good idea in EF. Ideally the OC should be kept alive for as short a time as possible.
Related
I'm using EF (EF Core, actually, with ASP.NET Core on OSX, but I believe this is more of a general "newbie-style" EF question, so please read on...)
I built a little logging routine that uses EF to publish log entries to my database. Sort of like this, called from a repository class:
WebLog log = new WebLog(source, path, message);
Context.WebLogs.Add(log);
Context.SaveChanges();
Where WebLog is a simple model class, Context.WebLogs is a DbSet<WebLog> collection, and Context is obviously the DbContext. I believe this is quite straightforward.
But my question is this: if I continue to add new log entries to the Context.WebLogs collection and I never do anything like reboot my server, isn't the collection just going to grow without bounds? Is there some kind of "purge" or "flush" action I can take periodically to manage memory usage (without affecting the committed rows in the database, of course--I want those to persist). Or is DbSet some sort of a special collection that won't do this?
As mentioned by DevilSuichiro above, the recommended approach is to limit the lifetime of the instances of DbContext. E.g. in a Web application you typically use a DbContext instance per request, so an unbounded number of entities added doesn't become a problem.
The closest thing to a "flush" operation is SaveChanges() that method will not try to remove references to tracked entities, as DbContext is designed to be reused after SaveChanges().
In previous versions of EF we had a Detach() API that you could use to get rid of an individual tracked reference but we don't have that API in DbContext or anywhere in EF Core.
BTW, having an instance of DbContext that is shared between multiple requests is extremely problematic because DbContext is not thread safe.
Hi
If anyone could please elaborate the difference between the three, i am new to EF and sometimes MergeOption.NoTracking happens to work whereas sometimes ObjectContext.Detach, but i never get the gist of it.
I would like to know Which situations should i use them. Also, if there is an object graph attached to some entity (either by firing the Include function or by calling EntityReference.Load()) What should be called if
1.) i don't want other objects attached to the entity
2.) i want all objects referenced by the entity
Any help would be greatly appreciated.
Regards
Hiren
MergeOption.NoTracking is used to improver performance for loading entities which will not be modified. Entity is in this case is not tracked by the context but it is still attached and lazy loading works.
ObjectContext.Detach completely removes entity from object context scope so the entity is not tracked and lazy loading doesn't work.
IEntityWithChangeTracker.SetChangeTracker is imho more like infrastructure for EntityObject. It is heavily used inside EF when entities are materialized and attached to the context.
I wonder if anyone can help me?
I am having problems understanding why i need to issues DetectChanges on my POCO (non proxy) entities.
Of course i have this line to ensure that proxies are not returned.
context.ObjectStateManager.GetObjectStateEntry(order).State
And doing some research it appears if i need to check the "state" of an object then i need to issue detechChanges But why would i need to check the State of an object?
Basically I send along my POCO entity to a method that SAVES the data to a new ObjectContext (I create and destroy ObjectContext on each method)
Hence, i am having problems understanding why i would need to have ObjectContext track or be aware of changes?
Is it because that if its not aware if will not be saved?
Maybe i am miss informed but it appears that if i am using an existing ObjectContext (which i am not i am creating and destroying each time) that ensure ObjectContext is aware would be beneficial but otherwise not?
So in 1 method I am updating an object by creating a new datacontext, saving it to the db and destroying ObjectContext . Hence i am not using 2 methods, 1 method to send the update or new record and then another method for SAVING.
I would really appreciate any quick explaanations of why its needed?
Thanks in advance
Your question is little bit confusing. You are writting about Entity Framework but using DataContext which is related to LinqToSql.
The behavior differs in the way you are using ObjectContext. When you load POCO entity from database ObjectContext holds its instance in internal Identity Map. By default POCO doesn't use any kind of change tracking. When you save that POCO entity to the same instance of ObjectContext it internally calls DetectChanges to compare current entity state with stored state. This comparision defines which columns have to be updated. Internal call to DetectChanges is default behavior which can be turned off so you will have to call this method manually.
In your scenario you not using the same instance of ObjectContext. In that case you first have to Attach POCO entity to the ObjectContext. MSDN strictly says that when attaching entity it is marked as Unchanged. For that reason you have to say ObjectContext that entity has changed. You can do that for whole entity or you can define exactly which properties have changed but you have to do it manually = you have to store that information somewhere (Self tracking entities can help you with that but they have ohter disadvantages).
In ADO.net EF, when should you call the context.Attach() and the context.Detach() methods and how do these calls affect the data being returned or being inserted?
This is one of those questions where, "If you have to ask, you probably should not be doing it." The Entity Framework will implicitly attach entities in cases where it is obvious that this needs to happen. You really only ever need to explicitly attach and detach entities in cases where you are using more than one ObjectContext at once. Because this can be quite confusing, due to the implicit attachment which happens in the course of normal Entity Framework operations, I strongly recommend that people new to the Entity Framework use only one ObjectContext at a time. If you do this, you should never need to explicitly call Attach or Detach.
Calling, say, Attach does not really affect the data returned, insofar as it's scaler properties are concerned. But if it refers to other entities which are already loaded into the context into which it is attached, then these properties will be pre-populated without explicit loading. That said, entities returned from a query are already attached, so you cannot attach them.
Attaching Objects (Entity Framework)
http://msdn.microsoft.com/en-us/library/bb896271.aspx
Detaching Objects (Entity Framework)
http://msdn.microsoft.com/en-us/library/bb738611.aspx
I'm new to the Entity Framework and am currently experimenting with it. I created a simple database, set up the model in VS2008, and have got the code going to query the database using the EF as well as inserting new data.
There's one thing that has me a little confused though. I have an entity (set up in my model) called Customer, and as part of the logic of my application I want to be able to create a temporary Customer object for some intermediate processing. This particular object should never actually be stored in the database. However, I noticed that as soon as I call SaveChanges() the customer is saved to the database. This isn't what I want to happen. I'd be quite happy to call AddCustomer() on the objects I do want to include - I just want to have the option to create a temporary instance for my own use.
I did discover I could call Detach() and pass in my temporary instance, which would stop it from being persisted. However I'm not sure this is the best way to do this since the temporary Customer object will have related objects, and unless I go through and detach them all I might end up in hot water.
It's possible I'm misunderstanding something about how the EF is supposed to work, or that I'm missing something obvious - I'm hoping someone can set me straight!
Thanks
John
If you want to have a temporary instance of an entity that'll never be connected to the EF again, use this Entity Cloner for cloning the entity
If you are trying to disconnect an entity, send it over the wire some where (let us say pass it over to the client over a service, to modify it, and then again get it back), and again merge back the changes to the EF - right now this is not directly supported. How ever, you can try these solutions
Entity Bag:
EFContrib (you need PostSharp4EF)
Why not have another Customer class with the same fields?
Just ran into this problem myself with a service using EF4 - there's a simpler solution - after you create the new entity instance, call
objectContext.Detach(newEntity);