Background:
I am using EF4 and ObjectContext. To optimze retrieval of complex object hierarchies, I manually execute database queries and then use ObjectContext.Translatey<T>(DataReader, entitySetName, mergeOptions.AppendOnly) to turn data rows into entities. I then attach the entities to the ObjectContext with Attach method. This also fixes relations between entities.
The problem:
After everything is loaded and set up I try to navigate from parent entity to a child entity (for example Parent.Childs.First()), but EF hits the database to load the kids, even though all the child entities are already present in the ObjectContext and EntitySet. It looks like the reason for this is that parent.Childs.IsLoaded is set to false which makes EF think that it still needs to load the relation.
Question:
How can I tell EF that EntitySet has already been loaded?
Is there a supported way to set RelatedEnd.IsLoaded to true. I wouldn't like to mess with the calling the RelatedEnd.SetIsLoaded internal method.
I found a smilar question here but it relates to DbContext and has no satifying answer ;-)
Matra
It looks like this was implemented in this change:
http://entityframework.codeplex.com/workitem/269
You can now iterate through your entities and tell them that their child collections are already loaded:
foreach (var entity in loadedEntities)
context.Entry(entity).Collection(a => a.SomeChildCollection).IsLoaded = true;
This will prevent entities in SomeChildCollection from being loaded when they are accessed from entity.
I'm not exactly sure what version of EF this appeared in but I'm guessing 6.0.0.
The only way to solve this is to turn off lazy loading. The question you have found on MSDN asks about DbContext but the answer mentions that there is no way to change the value in underlying libraries - ObjectContext API (= EF4 in your case) is the underlying library. Even in .NET 4.5 (EF5) setting IsLoaded is still not available on public API.
I am trying to add an entity to the DB. Once I have added it, I want to detach it, so I can manipulate the object safely without making any changes to the DB. After calling context.SaveChanges() I do the following to detach the entity:
// save
context.Stories.Add(story);
// attach tags. They already exists in the database
foreach(var tag in story.Tags)
context.Entry(tag).State = System.Data.EntityState.Unchanged;
context.SaveChanges();
context.Entry(story).State = System.Data.EntityState.Detached;
However, changing the entity state to DETACHED will remove all related entities associated with the my entity. Is there a way to stop this ?
If I don't detach the entity, all my changes are sent to the DB next time I call context.SaveChanges()
Thanks!!
There is no way. It is limitation of EF. Your options are:
Not using the same context for another save (single context instance = single save)
Retrieve the entity from database again using another context instance which will not be used for saving
Create deep clone of your entity and use the clonned one (deep clone is done by serialization and immediate deserialization = your entity graph must be serializable)
I think there are two ways to approach this problem:
Purist: retrieving entities from a DbContext and modifying them without saving is a misuse of the tools and the architecture. Use a DTO instead.
Pragmatic: you can use AsNoTracking() to retrieve an entity graph that will not be tracked by the context for changes.
I'm using MVC.net and I have 2 classes (case and accident) with a many to many relationship, I'm also using auto mapper to copy View Models to EF and vice versa. Now the problem i've come across is when i do this:
Case theCase = Mapper.Map<CaseEditVM, Case>(theCaseEditVM);
theCase.Accidents.Clear();
UOW.Cases.Update(theCase);
The changes to the case are saved but the link table for accidents is not. Ef totally ignores the Accidents changes.
However when i do:
Case theCase = UOW.Cases.GetByID(someid).Include("Accidents");
theCase.accidents.Clear();
UOW.Cases.Update(theCase);
EF correctly saves the accidents property.
So from what i can tell EF ignores the accident property as its not mapped inside EF yet. Make sense however how do i tell it when mapping the View model i want EF to update the linked properties as well?
The simplest way in your case is first attach case to context and clear changes after you attached it. Otherwise you will have a lot of work. There is no magic which would make this for you. EF doesn't know about changes to relations you did on detached entity and what is even worse once you attach the entity to context you already don't know what records were included in the navigation property so you cannot configure context to reflect that (it must be done per every single related entity) without reloading the whole entity and merging changes between detached and attached one.
I have an oData generated DataServiceContext and I am successfully adding entities to it. I need to add a whole load of entities and then commit them in a single SaveChanges with the Batch option set at the end. This is all fine, until I come to query it before the save changes.
Outline is:
Create a new entity
Add it to the DataServiceContext
Run a query on the context looking for the item I have just added - IT IS NOT FOUND
My previous work with EF4 would suggest that if this was an Entity Context, all would be fine, but because this is a Service Context I cannot query for an entity that has been added but not saved to the service.
Is this the case?
DataServiceContext is basically just a small helper. Running any query against it will run the query on the server directly, the client will not try to fixup the data in any way. Since you're changes haven't made it to the server yet (SaveChanges was not called yet), the query will not return the newly added entities.
If you really need to list the entities you've added before SaveChanges, you could use the DataServiceContext.Entities collection which will return EntityDescriptor for all entities tracked by the context. You can list those added by looking for those with state Added.
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}))