I have a core data entity which has an exclusive one to many relationship with another entity. This relationship is supposed to be a basic containment the first entity contains one or more of the second. An instance of the second entity can only be contained in one of the first entities.
I want to be able to remove all the contained entities from the first entity and then delete them. How should I do this? Should I remove the relationship and then call deleteObject for each entity or will calling deleteIObject for each contained entity cause the relationship to be set correctly. If the second is true, can I just enumerate the contained entities and call deleteObject or should I first make a copy of the set (if calling deleteObject for each objects severs the relationship this will modify the set which is not allowed in normal enumeration).
Delete each child object via the NSManagedObjectContext and the relationship will clean up appropriately. This is assuming that you are following the convention and have all of your relationships configured to be bi-directional.
Related
I have an entity that has a Set of child entities. Can I add/remove some of the children in the collection in Hibernate's org.hibernate.Interceptor.onFlushDirty()?
The documentation (javadoc, JBoss userguide) doesn't explicitly mention changing relationship state, so I just want to make sure it's legitimate.
Update
Decided to describe by problem.
There's a Parent entity that has a Set of Child entities. The Child entities are created based purely on fields of the Parent entity, no additional information needed. The children need to be updated each time before persisting the Parent.
I wanted to do it in Interceptor.onFlushDirty() by clearing existing children and adding new reclaculated (i.e. Transient) children, but I'm getting the TransientObjectException. As I understand, it's because I add transient Child entities during flush, but I'm not sure.
I have an entity(ex: Document) that is used as child in 4 other entities(using #OneToMany with #JoinTable in parents). I am not using Bidirectional Mapping. My requirement is to remove the Child(i.e Document), and I have two ways to do that, one way is, get the 4 parents, remove child from them and update them. Second, using native query(using jdbcTemplate) to remove entry from 4 join tables and remove the child.
Is there any other way it can be done in much simpler manner?
Create an abstract base class containing the Document as member and user JPA inhertiance --> http://en.wikibooks.org/wiki/Java_Persistence/Inheritance
Than it should be possible to get all users of a document with just one query.
Than it should be relatively easy to remove all references.
Don't do magic behind automatic deletion stuff. Thats for the cost of documentation.
Add orphan deletion (ie. delete child object when it's removed from collection in the parent). To enable it, you need to add
#OneToMany(orphanRemoval=true)
in owning entity.
I have 2 Entities, related by a many-to-many relationship.
Thing<<->>Tag
There is one NSArrayController controlling the entity "Tag", bound to the managedObjectContext. By the array controllers add: and remove: action i can add instances of tag to the collection.
There is a second NSArrayController controlling "Thing" entities, also bound to the managedObjectContext.
So each of the controllers manages all instances of their entity.
Now, let's say there are 5 "tag" and 3 "thing" instances already created by their array controllers.
I'd like to link individual tags to a thing. I just want to create the relationship between an existing thing to an existing tag instance.
Is addObject: of NSArrayController the right method for that? Or does it create a new managedObject?
Would it be equivalent to:
NSMutableSet *tags = [aThing mutableSetValueForKey:#"tags"];
[tags addObject:existingTag];
?
Is there some best practice for a tagging system?
I've found it helpful (in the latest version of Xcode) to select the entity in the core data modeller, and then go to the file menu, and select new file -> Core Data -> NSManagedObject subclass. It automatically creates a class with the necessary members AND ALSO methods for adding objects in the toMany relationships.
If you've done that, then you just need to get ahold of the thing instance to which you want to add a tag and you can call the method declared for you to do so. How that method is named is obvious from the header file generated.
I have three views that I've manually created in the DB.
First view is "Region", the second is "FIPS" and the last is a many-to-many between them called "Region2FIPS". These are all views, and I only need read access the data, so I'm not worried about having updateable views.
I have added each of these views to Entity Framework, and created the appropriate associations between them.
Region to Region2FIPS is a 1 to many.
FIPS to Region2FIPS is a 1 to many.
The "Region2FIPS" view contains only two columns, one called "FIPSID" the other "RegionID". These column are associated with their respective views in the relationships I defined above.
When this type of association is made on tables in the DB, Entity Framework knows that it is a many-to-many relationship and it creates a navigation property on "Region" called "FIPS" that I can use to navigate through the child collection of FIPS. It does likewise for "FIPS" to "Region".
However, when done manually, with views, it does not exhibit that behavior. Instead, my "Region" object has a collection of "Region2FIPS" objects, which each have a navigation property called "FIPS" which is of type "FIPS". And my "FIPS" object has a collection of "Region2FIPS" objects, which each have a navigation property called "Regions" of type "Region".
I assume this has something to do with the fact that I can't create foreign key references on the views, so entity framework doesn't realize the many-to-many relationship. But I thought that if I manually created the many-to-many relationship between the views it would recognize it and properly handle the navigation between the types. Is there a way for me to force it to do this?
It's possible, but the designer doesn't really help you here. You have to do the mapping manually.
One fairly easy way is to use Code First mapping. But this means your model has to be Code First to begin with. If you're writing a new model, just do that.
If you're using DB First mapping, however, you will have to do the mapping manually. Your SSDL will probably already be correct, once you define the "primary keys" of the views. You would then have to remove the "Region2FIPS" objects from the CSDL (not just from the designer!) and manually patch up the MSL.
Perhaps the easiest way to do this would be to use the designer to automatically map real DB tables (not views) with a similar schema and then replace the table names with view names in the EDMX, using the XML editor.
Do I need to save newly created CRM-entity instances before I can set relations to other crm entity instances?
I'm facing the problem that after calling CrmDataContext.SaveChanges() the newly created entities are written to the database, but the relations between those newly created instances are missing in the database.
What do I miss? Do I have to call CrmDataContext.SaveChanges() each time I create a new crm entity instance that I want to have relations to other CRM-entity instances?
You should be able to set relationships to other entities in a 1:N relationship before saving this entity(i.e. a lookup).
If you are wanting your entity to be referenced by another entity it will need to be Saved first OR you need to set a Guid for the entity first. Otherwise your link won't stick.
When you new up an entity its id is not set until it is saved to the database, unless you set it manually. If you set it manually it will respect the new Guid you have given it and the relationship will survive the saving process.
If you try to save an entity individually, you need to make sure that you have saved all the entities that it refers to before you save that entity, or it won't have a link.