I have recently changed from ObjectContext to DbContext using EntityFramwework by upgrading to EF6
Most stuff works, but saving and updating won't. Here is an example:
public void AssignToAdmin(Product product, Admin admin)
{
var pcsps = from s in context.ProductCreationSections
join pcsp in context.ProductCreationSectionAndProducts on s.ProductCreationSecitionID equals pcsp.ProductCreationSectionID
where pcsp.ProductID == product.ProductID && s.IsManagerAssigned
select pcsp;
foreach (var pcsp in pcsps.Include("AssignedAdmins"))
{
pcsp.AssignedAdmins.Add(admin);
}
}
Trying to execute the line pcsp.AssignedAdmins.Add(admin), I get the error:
Error: The relationship between the two objects cannot be defined
because they are attached to different ObjectContext objects.
There is one context for the class and it comes from Dependency Injection (the class is a Service in an MVC app).
I've tried removing/attaching and so on, but this doesn't fix it - it just gives different error messages. It's not even obvious which entity is using another context.
Any ideas of where this other context the error message refers to is coming from?
See The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects
Where has admin come from?
Getting the admin object from the same context as the pcsp object should help.
Sorted it, but it was quite a major refactor.
The issue was that each service received it's own instance of 'context', so when two entities from two services were expected to work together, they wouldn't as each belonged to a different context.
One solution would have been to make the class that created the context a 'Singleton' so that it always returned the same instance, but that would have been very BAD as every page would then use the same context.
Each service got it's own instance of 'context' through Dependency Injection.
I changed the project so only the controllers got an instance of context through DI. Then, in the controller constructor, this context was passed to the services that the controller had received through dependency injection.
This way, each request only ever uses a single instance of context, but this instance is still short lived, as it is supposed to be as each request has its own context and doesn't share one.
Not sure this is the way it is supposed to work, but given the web app I was working with, this seemed to be the best solution.
I also had to go through and add a 'context.SaveChanges();' statement everywhere a change to the database was made.
Not sure why this should be when the old version of EF did this automatically, but it now works.
Thanks for the advice Derrick
Related
I have a controller action where I am attempting to both create a new object in the database, of type FantasyTeam, and also modify an existing object, of type User. However, when the action is called I am receiving a System.InvalidOperationException exception, with the explanation of:
An entity object cannot be referenced by multiple instances of
IEntityChangeTracker.
My code is below.
public ActionResult Create([Bind(Include="ID,FantasyTeamName")] FantasyTeam fantasyteam)
{
if (ModelState.IsValid)
{
var CurrentUser = UserManager.FindById(User.Identity.GetUserId());
fantasyteam.OwnerID = CurrentUser.Id;
CurrentUser.HasTeam = true;
db.FantasyTeams.Add(fantasyteam);
db.Entry(CurrentUser).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("Dashboard", "Application");
}
return View(fantasyteam);
}
The Controller Action takes the new FantasyTeam object as a parameter from the View where the team details are selected by user. I then find the currently logged in user and set the OwnerID of the team to match the UserID of the owner, as my Foreign Key. I then go to add the new FantasyTeam to the database and also modify the User record in the database, changing the Boolean HasTeam to true. The adding of the new team works fine on its own... I have been getting the exception once I tried to modify the User object as well.
I have searched on StackOverflow for this exception, and have found many related issues, but I couldn't find a way to apply the other resolutions to my problem. In some cases the issue was that the developers were using two separate data contexts when they should have just been using one, but I don't think that would apply to me here.
Could someone offer insight as to what might be going wrong here?
Thanks!
According to Linq to SQL DataContext Lifetime Management the main causality of the problem comes from the below reason.
Linq to SQL uses a DataContext to manage it's access to the database as well as tracking changes made to entities retrieved from the database. Linq to SQL has a persistent approach to managing its 'connection' to the database via this data context and it basically assumes that you use a single DataContext to make all of your data related access. This doesn't mean that it makes persistent connections to the database, but means that the DataContext instance maintains state about active result sets, which is especially important if change tracking is on which is the default.
The comment posted by haim770 was the answer - I didn't realize that the UserManager object was using a different context than the db object was using. Once I changed the UserManager object to use the same context as db, my code works as intended.
The objective of the method below is to delete the data from the database using entity framework. I am populating the subscriptions entity by passing two parameters and then calling the atttach and remove method of the context class. Bureau entities is the context class.
using (BUREAUEntities bureauEntities = new BUREAUEntities())
{
var subscription = new SubcriptionRepository().GetSusbcriptionBySubscriptionTypeId(subscriptionTypeId, companyId);
bureauEntities.Subscriptions.Attach((DataAccessLayer.DatabaseModel.Subscription)subscription);
bureauEntities.Subscriptions.Remove(subscription);
bureauEntities.SaveChanges();
}
I am getting the following error message at An entity object cannot be referenced by multiple instances of IEntityChangeTracker at line
bureauEntities.Subscriptions.Attach((DataAccessLayer.DatabaseModel.Subscription)subscription);
Could somebody tell me where am I going wrong ?
Generally, the error means that you retrieved subscription from one instance of your BUREAUEntities context, while you're trying to remove it from another instance. The cure is to simply be smart about instantiating your context, and it's easy to fall down on that front when using using, since it disposes of the instance as soon as the block exits.
Essentially, make sure all your code that will interact with your context is within the same using block. Even better, use a DI (dependency injection) container like Ninject or Unity to create a context scoped to the request. Then you never have to worry about ending up with multiple copies of the same context floating around.
I'm learning ASP.NET MVC and I'm having some questions that the tutorials I've read until now haven't explored in a way that covers me. I've tried searching, but I didn't see any questions asking this. Still, please forgive me if I have missed an existing ones.
If I have a single ASP.NET MVC application that has a number of models (some of which related and some unrelated with each other), how many DbContext subclasses should I create, if I want to use one connection string and one database globally for my application?
One context for every model?
One context for every group of related models?
One context for all the models?
If the answer is one of the first two, then is there anything I should have in mind to make sure that only one database is created for the whole application? I ask because, when debugging locally in Visual Studio, it looks to me like it's creating as many databases as there are contexts. That's why I find myself using the third option, but I'd like to know if it's a correct practice or if I'm making some kind of mistake that will come back and bite me later.
#jrummell is only partially correct. Entity Framework will create one database per DbContext type, if you leave it to its own devices. Using the concept of "bounded contexts" that #NeilThompson mentioned from Julie Lerhman, all you're doing is essentially telling each context to actually use the same database. Julie's method uses a generic pattern so that each DbContext that implements it ends up on the same database, but you could do it manually for each one, which would look like:
public class MyContext : DbContext
{
public MyContext()
: base("name=DatabaseConnectionStringNameHere")
{
Database.SetInitializer(null);
}
}
In other words, Julie's method just sets up a base class that each of your contexts can inherit from that handles this piece automatically.
This does two things: 1) it tells your context to use a specific database (i.e., the same as every other context) and 2) it tells your context to disable database initialization. This last part is important because these contexts are now essentially treated as database-first. In other words, you now have no context that can actually cause a database to be created, or to signal that a migration needs to occur. As a result, you actually need another "master" context that will have every single entity in your application in it. You don't have to use this context for anything other than creating migrations and updating your database, though. For your code, you can use your more specialized contexts.
The other thing to keep in mind with specialized contexts is that each instantiation of each context represents a unique state even if they share entities. For example, a Cat entity from one context is not the same thing as a Cat entity from a second context, even if they share the same primary key. You will get an error if you retrieved the Cat from the first context, updated it, and then tried save it via the second context. That example is a bit contrived since you're not likely to have the same entity explicitly in two different contexts, but when you get into foreign key relationships and such it's far more common to run into this problem. Even if you don't explicitly declare a DbSet for a related entity, it an entity in the context depends on it, EF will implicitly create a DbSet for it. All this is to say that if you use specialized contexts, you need to ensure that they are truly specialized and that there is zero crossover at any level of related items.
I use what Julie Lerman calls the Bounded Context
The SystemUsers code might have nothing to do with Products - so I might have a System DbContext and a Shop DbContext (for example).
Life is easier with a single context in a small app, but for larger application it helps to break the contexts up.
Typically, you should have one DbContext per database. But if you have separate, unrelated groups of models, it would make sense to have separate DbContext implementations.
it looks to me like it's creating as many databases as there are
contexts.
That's correct, Entity Framework will create one database per DbContext type.
I have two databases that I am accessing. The first is against a contact database which I connected to using EF Model First; creating the edmx. I have since begun to learn the virtue of CODE First when working with Entity Framework, so I decided I would, in the same project, write the Product database using Code First techniques, allowing the database to be generated from the code I am writing.
Everything compiles fine. The problem occurs when I hit my harness and it attempts to create the Product database and retreive a list of values from one of the tables...
I get the folowing error "Could not find the conceptual model type for 'Core.Data.Account'", when I attempt to enumerate the ProductLines property (Line3 below).
1. using (var ctx = new ProductDb())
2. {
3. var lines = ctx.ProductLines.ToList();
4. this.litOne.Text = lines.Count.ToString();
5. }
After some research it appears that this message may be occuring because of multiple entities with the same name (regardless of namespace), however there is nothing in the ProductDb context with the name "Account".
There is a class in the OTHER context created by the Model First approach named "Account". But how/why would that make a difference? They each point to different databases i.e. different connection strings. Why would the ProductDb be attempting to create a table called Account, when it should be completely unaware of it's exstence?
thoughts?
Thank you as always!,
- G
I bumped into the same problem, but the other way around: first a DbContext + generated database and then generated an edmx off the database (just for a little presentation). It appeared to be a restriction in EF: EF currently has a restriction that POCO classes can't be loaded from an assembly that contains classes with the EF attributes.
The only thing you can do for now is keep the contexts in separate assemblies.
Entity Framework 4, POCO objects and ASP.Net MVC2. I have a many to many relationship, lets say between BlogPost and Tag entities. This means that in my T4 generated POCO BlogPost class I have:
public virtual ICollection<Tag> Tags {
// getter and setter with the magic FixupCollection
}
private ICollection<Tag> _tags;
I ask for a BlogPost and the related Tags from an instance of the ObjectContext and send it to another layer (View in the MVC application). Later I get back the updated BlogPost with changed properties and changed relationships. For example it had tags "A" "B" and "C", and the new tags are "C" and "D". In my particular example there are no new Tags and the properties of the Tags never change, so the only thing which should be saved is the changed relationships. Now I need to save this in another ObjectContext. (Update: Now I tried to do in the same context instance and also failed.)
The problem: I can't make it save the relationships properly. I tried everything I found:
Controller.UpdateModel and Controller.TryUpdateModel don't work.
Getting the old BlogPost from the context then modifying the collection doesn't work. (with different methods from the next point)
This probably would work, but I hope this is just a workaround, not the solution :(.
Tried Attach/Add/ChangeObjectState functions for BlogPost and/or Tags in every possible combinations. Failed.
This looks like what I need, but it doesn't work (I tried to fix it, but can't for my problem).
Tried ChangeState/Add/Attach/... the relationship objects of the context. Failed.
"Doesn't work" means in most cases that I worked on the given "solution" until it produces no errors and saves at least the properties of BlogPost. What happens with the relationships varies: usually Tags are added again to the Tag table with new PKs and the saved BlogPost references those and not the original ones. Of course the returned Tags have PKs, and before the save/update methods I check the PKs and they are equal to the ones in the database so probably EF thinks that they are new objects and those PKs are the temp ones.
A problem I know about and might make it impossible to find an automated simple solution: When a POCO object's collection is changed, that should happen by the above mentioned virtual collection property, because then the FixupCollection trick will update the reverse references on the other end of the many-to-many relationship. However when a View "returns" an updated BlogPost object, that didn't happen. This means that maybe there is no simple solution to my problem, but that would make me very sad and I would hate the EF4-POCO-MVC triumph :(. Also that would mean that EF can't do this in the MVC environment whichever EF4 object types are used :(. I think the snapshot based change tracking should find out that the changed BlogPost has relationships to Tags with existing PKs.
Btw: I think the same problem happens with one-to-many relations (google and my colleague say so). I will give it a try at home, but even if that works that doesn't help me in my six many-to-many relationships in my app :(.
Let's try it this way:
Attach BlogPost to context. After attaching object to context the state of the object, all related objects and all relations is set to Unchanged.
Use context.ObjectStateManager.ChangeObjectState to set your BlogPost to Modified
Iterate through Tag collection
Use context.ObjectStateManager.ChangeRelationshipState to set state for relation between current Tag and BlogPost.
SaveChanges
Edit:
I guess one of my comments gave you false hope that EF will do the merge for you. I played a lot with this problem and my conclusion says EF will not do this for you. I think you have also found my question on MSDN. In reality there is plenty of such questions on the Internet. The problem is that it is not clearly stated how to deal with this scenario. So lets have a look on the problem:
Problem background
EF needs to track changes on entities so that persistance knows which records have to be updated, inserted or deleted. The problem is that it is ObjectContext responsibility to track changes. ObjectContext is able to track changes only for attached entities. Entities which are created outside the ObjectContext are not tracked at all.
Problem description
Based on above description we can clearly state that EF is more suitable for connected scenarios where entity is always attached to context - typical for WinForm application. Web applications requires disconnected scenario where context is closed after request processing and entity content is passed as HTTP response to the client. Next HTTP request provides modified content of the entity which has to be recreated, attached to new context and persisted. Recreation usually happends outside of the context scope (layered architecture with persistance ignorace).
Solution
So how to deal with such disconnected scenario? When using POCO classes we have 3 ways to deal with change tracking:
Snapshot - requires same context = useless for disconnected scenario
Dynamic tracking proxies - requires same context = useless for disconnected scenario
Manual synchronization.
Manual synchronization on single entity is easy task. You just need to attach entity and call AddObject for inserting, DeleteObject for deleting or set state in ObjectStateManager to Modified for updating. The real pain comes when you have to deal with object graph instead of single entity. This pain is even worse when you have to deal with independent associations (those that don't use Foreign Key property) and many to many relations. In that case you have to manually synchronize each entity in object graph but also each relation in object graph.
Manual synchronization is proposed as solution by MSDN documentation: Attaching and Detaching objects says:
Objects are attached to the object
context in an Unchanged state. If you
need to change the state of an object
or the relationship because you know
that your object was modified in
detached state, use one of the
following methods.
Mentioned methods are ChangeObjectState and ChangeRelationshipState of ObjectStateManager = manual change tracking. Similar proposal is in other MSDN documentation article: Defining and Managing Relationships says:
If you are working with disconnected
objects you must manually manage the
synchronization.
Moreover there is blog post related to EF v1 which criticise exactly this behavior of EF.
Reason for solution
EF has many "helpful" operations and settings like Refresh, Load, ApplyCurrentValues, ApplyOriginalValues, MergeOption etc. But by my investigation all these features work only for single entity and affects only scalar preperties (= not navigation properties and relations). I rather not test this methods with complex types nested in entity.
Other proposed solution
Instead of real Merge functionality EF team provides something called Self Tracking Entities (STE) which don't solve the problem. First of all STE works only if same instance is used for whole processing. In web application it is not the case unless you store instance in view state or session. Due to that I'm very unhappy from using EF and I'm going to check features of NHibernate. First observation says that NHibernate perhaps has such functionality.
Conclusion
I will end up this assumptions with single link to another related question on MSDN forum. Check Zeeshan Hirani's answer. He is author of Entity Framework 4.0 Recipes. If he says that automatic merge of object graphs is not supported, I believe him.
But still there is possibility that I'm completely wrong and some automatic merge functionality exists in EF.
Edit 2:
As you can see this was already added to MS Connect as suggestion in 2007. MS has closed it as something to be done in next version but actually nothing had been done to improve this gap except STE.
I have a solution to the problem that was described above by Ladislav. I have created an extension method for the DbContext which will automatically perform the add/update/delete's based on a diff of the provided graph and persisted graph.
At present using the Entity Framework you will need to perform the updates of the contacts manually, check if each contact is new and add, check if updated and edit, check if removed then delete it from the database. Once you have to do this for a few different aggregates in a large system you start to realize there must be a better, more generic way.
Please take a look and see if it can help http://refactorthis.wordpress.com/2012/12/11/introducing-graphdiff-for-entity-framework-code-first-allowing-automated-updates-of-a-graph-of-detached-entities/
You can go straight to the code here https://github.com/refactorthis/GraphDiff
I know it's late for the OP but since this is a very common issue I posted this in case it serves someone else.
I've been toying around with this issue and I think I got a fairly simple solution,
what I do is:
Save main object (Blogs for example) by setting its state to Modified.
Query the database for the updated object including the collections I need to update.
Query and convert .ToList() the entities I want my collection to include.
Update the main object's collection(s) to the List I got from step 3.
SaveChanges();
In the following example "dataobj" and "_categories" are the parameters received by my controller "dataobj" is my main object, and "_categories" is an IEnumerable containing the IDs of the categories the user selected in the view.
db.Entry(dataobj).State = EntityState.Modified;
db.SaveChanges();
dataobj = db.ServiceTypes.Include(x => x.Categories).Single(x => x.Id == dataobj.Id);
var it = _categories != null ? db.Categories.Where(x => _categories.Contains(x.Id)).ToList() : null;
dataobj.Categories = it;
db.SaveChanges();
It even works for multiple relations
The Entity Framework team is aware that this is a usability issue and plans to address it post-EF6.
From the Entity Framework team:
This is a usability issue that we are aware of and is something we have been thinking about and plan to do more work on post-EF6. I have created this work item to track the issue: http://entityframework.codeplex.com/workitem/864 The work item also contains a link to the user voice item for this--I encourage you to vote for it if you have not done so already.
If this impacts you, vote for the feature at
http://entityframework.codeplex.com/workitem/864
All of the answers were great to explain the problem, but none of them really solved the problem for me.
I found that if I didn't use the relationship in the parent entity but just added and removed the child entities everything worked just fine.
Sorry for the VB but that is what the project I am working in is written in.
The parent entity "Report" has a one to many relationship to "ReportRole" and has the property "ReportRoles". The new roles are passed in by a comma separated string from an Ajax call.
The first line will remove all the child entities, and if I used "report.ReportRoles.Remove(f)" instead of the "db.ReportRoles.Remove(f)" I would get the error.
report.ReportRoles.ToList.ForEach(Function(f) db.ReportRoles.Remove(f))
Dim newRoles = If(String.IsNullOrEmpty(model.RolesString), New String() {}, model.RolesString.Split(","))
newRoles.ToList.ForEach(Function(f) db.ReportRoles.Add(New ReportRole With {.ReportId = report.Id, .AspNetRoleId = f}))