I need to be able to create new core data entities during runtime. I've written the code to create the objects programmatically, however, I can't add the entities during runtime as the model is immutable.
My problem is similar to this post, however there is no satisfactory answer: How to dyanmic create a new entity (table) via CoreData model?
The documentation regarding changing the core data model explains:
Managed object models are editable until they are used by an object
graph manager (a managed object context or a persistent store
coordinator). This allows you to create or modify them dynamically.
However, once a model is being used, it must not be changed. This is
enforced at runtime—when the object manager first fetches data using a
model, the whole of that model becomes uneditable. Any attempt to
mutate a model or any of its sub-objects after that point causes an
exception to be thrown. If you need to modify a model that is in use,
create a copy, modify the copy, and then discard the objects with the
old model.
However, I'm unclear on what exactly this is saying--that the whole core data model can't be changed once the persistent store coordinator has been used or the attributes/etc of the individual entities can't be changed.
To be clear, I do not want to change the attributes of my current entities, I simply want to add new entities. It just seems weird to me to have to use migration to add new entities.
Any thoughts?
Thanks!
The documentation is pretty clear.
Copy the model.
Apply your changes to the new copy.
Destroy your old MOC, Persistent Store Coordinator, and all objects created from those.
Apply a migration, if necessary.
Create a new Core Data Stack (MOC, PSC, etc) using your updated model.
The migration could be a sticking point, but it should be do-able.
Related
I have a database which consists of 3 entities, say
'IronMan', 'CaptainAmerica' and 'SpiderMan'.
With new changes, I want to delete all three entities and create another entity called 'Thanos'.
I would not need to use any of the code and data stored earlier with entities 'IronMan', 'CaptainAmerica' and 'SpiderMan'. Do I need to do core data versioning or migration in this case?
As mentioned in Apple's documentation
https://developer.apple.com/documentation/coredata/using_lightweight_migration
You can add, remove, and rename entities in the hierarchy.
So, yes, you need to provide migration (new version model) but can stick to a light-weight migration, hence it will be done automatically based on changes done between the 2 models.
Recently, I am trying to refine my code. I found that all my entities has attributes named identifier and owner, so I want to create a entity to be their parent which contains identifier and owner.
Following is the result, all object inherit from a parent named SRModel, which has identifier and owner attributes.
However, after I delete all these redundant properties, the persistent store is not able to auto migration.
How can I solve the problem? Do I have to do migration by myself?
Are there any simple way to do so?
According to Apple's Core Data Model Versioning and Data Migration Programming Guide, you can't do that automatically.
You cannot, however, merge entity hierarchies; if two existing entities do not share a common parent in the source, they cannot share a common parent in the destination
Note Andy Riordan's point about inheritance. And don't just take his word for it; look at the generated .SQLite files yourself under the old and new models. Adding a parent entity with only two common attributes will just make your entities, and backing tables, larger, with no performance benefit. If you really want to note the two common elements, use a Protocol to call them out.
I am learning a bit on NSCoreData and before introducing it some existing projects I have, I would like to validate my good understanding of the core principles.
From what I have understood, NSCoreData make it easier to manage local storage of object (+retrieval after that) by subclassing our Model class from NSManagedObject rather than from NSObject.
Right ?
I have a few questions then. Let's consider I am building a real estate application with as core model object the class Property that can represent an appartment, a house, and all related information. Currently it is managed in my app as a subclass of NSObject.
1) I retrieve the properties from the server through a search query, and have written a initWithJson : method to populate each instance.
Now if I subclass Property from NSManagedObject, I will create my instances by using
+(id)insertNewObjectForEntityForName:(NSString *)entityName
inManagedObjectContext:(NSManagedObjectContext *)context
and I will be still be able to add a populateWithJson: to my class to fill in the properties.
Then I will create a lot of Property instances in the current managedObjectContext, and if I do a save, they will be stored at the physical layer.
If I call again the same webservice, and retrieve the same JSON content, I will recreate the identical managed objects.
How to avoid redundancy with the [managedObjectContext save:&error] call and not to store physically several time the representation of a single real life property ?
2) Let's say I want to store physically only some properties, for instance only the one the user want to have as favorites.
[managedObjectContext save:&error] will save all created / modified / deleted managed objects from the context to the physical layer, and not only the one I want.
How to achieve that ?
Am I supposed to declare another context (managedObjectContext2), move the instance I want to store in that context, and do the save in that one ?
(I mean, I will have a context just to manipulate the object, create instances from the JSON and represents them in UI ... and a second one to actually do the storage)
Or am I supposed to stores all the objects, and add a isFavorite BOOL property , and then fetching using a predicate on that property ?
3) The app has a common navigation pattern : the UITableView lists Properties instance with the minimum information required, and going on a detail view call a webservice to request more information on a specific Property instance (images, full text description).
Is it a good practice for instance to call the webservice only if the property.fullDescription is nil, and then update the object and store it locally with all detailed information, and the next time only to fetch it locally with a predicate on the property.id ?
What about object that might be updated server-side after they have been created?
Thanks for your lights
1) Retrieve the server data into a temporary form (array of dictionaries?), then for each possible property in the array, check to see if you already have an object in Core Data that matches. If you do, either ignore it or update any changed attributes; if not, create a Property object.
2) Decide which things you want to persist in order to support your app's functions. There's no point in creating a managed object for something you don't want to save. Note, though, that Core Data supports sub-classes if you want both Property and FavoriteProperty.
3) Entirely up to your "business rules"…. How often do you need local data to be updated? The only technical consideration might be the guideline to not keep large files locally that can be re-created on demand.
After reading RestKit docs about RKManagedObjectStore I was confused about createPersistentStoreCoordinator method because there is a next warning in the description of this method:
**Warning:** Creating the persistent store coordinator will
render the managed object model immutable. Attempts to
use functionality that requires a mutable managed object model
after the persistent store coordinator has been created
will raise an application error.
I didn't understant what does it mean immutable managed object model? I can't found any information about this topic neither in official Core Data docs nor accross the Internet. Can someone give me an explanation of the difference between mutable and immutable managed object models? Why does creating of persistent store coordinator renders immutable managed object model? And what functionality requires a mutable managed object model?
Thanks in advance.
You can change a NSManagedObjectModel (add entities and attributes for example) in code, it is said to be mutable. But once you attach your object model to an persistent store coordinator you are not allowed to change it anymore - it has become immutable.
This also is described in the documentation for NSManagedObjectModel:
Editing Models Programmatically Managed object models are editable
until they are used by an object graph manager (a managed object
context or a persistent store coordinator). This allows you to create
or modify them dynamically. However, once a model is being used, it
must not be changed. This is enforced at runtime—when the object
manager first fetches data using a model, the whole of that model
becomes uneditable. Any attempt to mutate a model or any of its
sub-objects after that point causes an exception to be thrown. If you
need to modify a model that is in use, create a copy, modify the copy,
and then discard the objects with the old model.
I'm learning Core Data and I understand all the examples for creating a brand-new object, assigning values and saving it to the managedContext (insertNewObjectForEntityForName).
However, what if I've already created an object elsewhere (model Category)? In this case I'd want to just assign the current Context to this Model, and then save it.
What is the command/approach to take an in-memory Model, and then assign to a context so it can be saved?
If you want to use Core Data to manage your data, you'll need to:
create an appropriate model description (.xcdatamodeld file)
modify your model class(es) so that they inherit from NSManagedObject
set the "Class" for each entity in your model description to one of your NSManagedObject subclasses
add code to your app to create and manage the Core Data stack, fetch data, etc.
This is all very do-able, but I wouldn't recommend that you attempt it until you have a solid understanding of Core Data and your reasons for adopting it in your project. The lack of clarity in your question may indicate that you're not quite there yet; you might benefit from working on a small project that uses Core Data from the start.
If all you want to do is to save your data, you should know that Core Data is not the only way to do that. A much simpler approach to saving your data would be to adopt the NSCoding protocol in your data model and then use a NSKeyedArchiver to store your data. Get the full story from the Archives and Serializations Programming Guide. There are other ways to do it as well, but NSKeyedArchiver is a good place to start.
You can only save NSManagedObjects (and their subclasses) to CoreData. NSManagesObject can not be created except in the context of an NSManagedObjectContext.
So, what you're saying is confusing. Do you have a non-CoreData model object?