Update relationships when saving changes of EF4 POCO objects

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:

1. Save main object (Blogs for example) by setting its state to Modified.
2. Query the database for the updated object including the collections I need to update.
3. Query and convert .ToList() the entities I want my collection to include.
4. Update the main object's collection(s) to the List I got from step 3.
5. 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}))