I have a core data design with multiple tables using relationships. My database is SQLite. For updates I import data from JSON and use this method:
[NSEntityDescription insertNewObjectForEntityForName:entityName inManagedObjectContext:context].
I have added unique constraints in core data.
If I update an entity that is a relationship of another object it loses the connection.
Ex: Entity "person" that contains the one to one relationship to "pet_id". If I update "pet" it changes his id and "person" still points to the old id, so they are not related any more.
Is there a way to avoid this problem?
I don't think this is documented anywhere yet. Here's what it sounds like is happening:
You create a new instance. Your constraints mean that this instance matches an existing instance. But...
Your new instance has a nil value for this relationship. So...
The existing instance's value for the relationship is replaced by this new nil value.
To maintain the relationship, your new instance needs to already have the correct value for that relationship. You're essentially asking that the constraint matching system ignore the fact that the relationship value is different in your new instance, but to accept new values for other attributes.
I think what you're expecting is completely reasonable but I'm also not surprised that the current implementation doesn't support it. I recommend filing a bug with Apple about this, and investigating non-constraint based approaches to keeping your data unique.
Related
I'm in the process of removing RestKit from our iOS app. I'm able to get things that I want into Core Data, but they're not really connected.
For example, we have one network call that returns a list of "Category"s (which have a "categoryID" and a "categoryName"; "Category"s also map to-many "StoreLocation"s). We then have another network call that returns a list of "StoreLocation"s (which, among other things, have a "storeName", "storeID", "storeCategoryIDs"; "StoreLocation"s also map to-many "Category"s).
With RestKit, I could use a RKConnectionDescription to describe that "storeCategoryIDs" drove the relationship to-many "Category"s. With that, if I had a given Category object, I could easily determine which StoreLocations belonged to that category.
I'm struggling to see how to accomplishing this without any RestKit dependencies. I suppose I could, whenever I'm about to insert a new Category or new StoreLocation, fetch all of the opposite managed objects and do this manually, but I seem to be missing some component of Core Data that can do.
The main part you're missing is the predicate applied to the fetch and which uses the identification attributes to find the appropriate existing objects. You do need to run your own fetch as core data will not magically update one object if you create a different new object and insert it.
I am using coredata to save the server data through web services in my application and I am storing relationships as an object to the entity.
I have many entities e.g "Inspirations" and "Products" and both are related to each other. I have a problem whenever the records are updated in the third entity which is "Filters" then the relations of the entities broke and I cannot apply filters on the entities.
[object addRelatedInspirationsObject:related];
This is how I save relationships. I am not able to figure out why the relations are being broken once the entity is updated which has no direct link with the entity.
One thing more if I fetch and save the data of any one of the entities like "Inspirations" then all the relations start to work again.
Your code should work. Here are 2 things you need to check:
Make sure related is not nil when you call your method.
Make sure you call save on a valid managed object context.
From your question it seems that entities have 1 to many relationship between them. And by the code you supplied, every things should work fine. Just make sure, you are using the Filter object from the relationship like object.filter (or obj1.obj2.filter), not accessing it via a direct NSPredicate on Filter entity and updating it. And if you are using FRC, you might also need to generate a fault against the parent entities, to get your UI updates.
When I retrieve an entity from the one side of a one-to many relationship, I create a mutable array from the set that is the collection of entities from the relationship. I manipulate, edit or otherwise change those entities, possibly delete existing or add new.
When through with the changes I simply use the array to create a new set then replace the original set with that which I created like so:
self.myOneSideEntity.theManySideEntitiesRelationship = [NSSet setWithArray:myNewArrayOfEntities];
It occurred to me that replacing the set may not be deleting the old members. What happened to them? Is this the proper way to edit the collection of related objects? Am I leaving any kind of orphans or going against best practices with this technique?
My relationship is set up with an inverse, cascade delete on the one side, nullify on the many side and the inverse relationship is not optional.
I've spent some days to understand similar behavior in my application.
Relation's "Delete Rule" works only when the object that contains relation is deleted itself. If you simply replace one set of objects with another (as you do) - nothing happens. Child objects that were in old set will simply have inverse relations set to nil. So if that relation (from child side) is not optional, you will get CoreData error when saving context.
For now I didn't find any way to manage this, except manual deletion of old objects.
For me the issue was with getting objects which were wired with current object. (groupObject.docs)
It was solved when I add context by which I get this data.
I'm using MagicalRecord:
[GroupObject MR_findAllInContext:[NSManagedObjectContext MR_defaultContext]]
instead of
[GroupObject MR_findAll]
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}))
I'm implementing core data in my iphone app. It has two entities.
Entity1: LatestData
Entity2: LatestDetailedData
LatestData has URL, publishedDate, heading
LatestDetailedData has URL, NewsDescription, PublishedDate, Author
Both entities have same URL for a record.
Both the entities are connected with inverse relation ship. And the relation ship is "delete->Cascaded"
What I want: If I remove a record in LatestData, I want the record with same URL in LatestDetailedData must also be deleted.
How?
If I am understanding you correctly, you are using a relationship and it has an inverse. If that is the case then when you delete one then Core Data will automatically delete the other and you don't need to do anything extra.
What are you seeing that suggests that is not happening?
Update
Since you are using multiple threads, are you using one NSManagedObjectContext per thread? If so, are you updating all of the threads when a save occurs? I suspect one of those two is not occurring and therefore causing your issue.
when you create the entities you need to create the relationship too
LatestDetailedData * entity2 = [[NSEntityDescription insertNewObjectForEntityForName:#"LatestDetailedData" inManagedObjectContext:context];
entity1.lastestdetail=entity2;
if you are just relying on a URL field, then thats bad practice. Set up the relationship in coredata and the cascaded deletes will take care of themselves.