TL;DR: Is there a way to programmatically read/recall (NOT write!) an instance of a Core Data entity using the p-numbered "serial number" that's tacked on to the instance's x-coredata:// identifier? Is this a good/bad idea?
I'm using a method similar to the following to retrieve the instances of an Entity called from a Core Data data store:
var managedContext: NSManagedObjectContext!
let fetchRequest : NSFetchRequest<TrackInfo> = TrackInfo.fetchRequest()
fetchResults = try! managedContext.fetch(fetchRequest)
for (i, _) in Global.Vars.numberOfTrackButtons! {
let workingTrackInfo = fetchResults.randomElement()!
print("current track is: \(workingTrackInfo)")
The list of tracks comes back in fetchResults as an array, and I can select one of them at random (fetchResults.randomElement()). From there, I can examine the details of that one item by coercing it to a string and displaying it in the console (the print statement). I don't list the code below, but using workingTrackInfo I am able to see that instance, read its properties into other variables, etc.
In the console, iOS/Xcode lists the selected item as follows:
current track is: <MyProjectName.TrackInfo: 0x60000374c2d0> (entity:
TrackInfo; id: 0xa7dc809ab862d89d
<x-coredata://2B5DDCDB-0F2C-4CDF-A7B9-D4C43785FDE7/TrackInfo/p22>;
data: <fault>)
The line beginning with x-coredata: got my attention. It's formatted like a URL, consisting of what I assume is a UUID for the specific Core Data store associated with the current build of the app (i.e. not a stable address that you could hardcode; you'd need to programmatically look up the Core Data store, similar to the functions we use for programmatically locating the Documents Folder, App Bundle, etc.) The third item is the name of the Entity in my Core Data model -- easy enough.
But that last number is what I'm curious about. From examining the SQLite database associated with this data store, it appears to be a sort of "instance serial number" associated with the Z_PK field in the data model.
I AM NOT interested in trying to circumvent Core Data's normal mechanisms to modify the contents of a managed object. Apple is very clear about that being a bad idea.
What I AM interested in is whether it's possible to address a particular Core Data instance using this "serial number".**
In my application, where I'm randomly selecting one track out of what might be hundreds or even thousands of tracks, I'd be interested in, among other things, the ability to select a single track on the basis of that p-number serial, where I simply ask for an individual instance by generating a random p-number, tack it on to a x-coredata:// statement formatted like the one listed above, and loading the result (on a read-only basis!) into a variable for further use elsewhere in the app.
For testing purposes, I've tried simply hardcoding x-coredata://2B5DDCDB-0F2C-4CDF-A7B9-D4C43785FDE7/TrackInfo/p22 as a URL, but XCode doesn't seem to like it. Is there some other data Type (e.g. an NSManagedObject?) that allows you to set an x-coredata:// "URL" as its contents?
QUESTIONS: Has anyone done anything like this; are there any memory/threading considerations why grabbing instance names in this manner is a bad idea (I'm an iOS/Core Data noob, so I don't know what I don't know; please humor me!); what would the syntax/method for these types of statements be?
Thanks!
You are quite close.
x-coredata://2B5DDCDB-0F2C-4CDF-A7B9-D4C43785FDE7/TrackInfo/p22
is the uriRepresentation() of the NSManagedObjectID of the record.
You get this URL from an NSManagedObject with
let workingTrackInfo = fetchResults.randomElement()!
let objectIDURL = workingTrackInfo.objectID.uriRepresentation()
With this URL you can get the managed Object ID from the NSPersistentStoreCoordinator and the coordinator from the managed object context.
Then call object(with: on the context to get the object.
let persistentStoreCoordinator = managedContext.persistentStoreCoordinator!
if let objectID = persistentStoreCoordinator.managedObjectID(forURIRepresentation: objectIDURL) {
let object = managedContext.object(with: objectID) as! TrackInfo
print(object)
}
Related
Im trying to find the best way to get the latest entity from a relationship with swift and core data.
For example I have a Conversation with many Messages:
Conversation Entity:
id = 1, name = "Test"
Message Entity:
id = 1, body = "Test", conversation = 1
id = 2, body = "Test", conversation = 1
In my SwiftUI View I have this FetchRequest:
#FetchRequest(sortDescriptors: []) var conversations: FetchedResults<Conversation>
What would be the best way to access the relation and get the latest result while looping over the conversations in View like this:
List(conversations) { conversation in
Text(conversation.name ?? "")
Text(conversation.messages.latest.body ?? "") // <--- Trying to do something like this
}
The best way is to store the data you need in the entity you are fetching. So in your case make a latestMessageBody property on the conversation entity. Apple provide derived attributes to automate this.
It's a common mistake to think of Core Data as a relational database, instead just think of it as a way to persist what you see on screen and design your entities based on how you want to fetch and display them. If you are making too many fetches on one screen then its time to redesign the entities.
I.e. you wouldn't want to fetch the latest message for all the converstations you want to show that is too many fetches. And you don't want to load in all the messages for every conversation just to find the latest, that has too high memory overhead. This is why Core Data offers denormalisation via the derived attributes feature.
Another point is #FetchRequest (which is powered by NSFetchedResultsController by the way) only tracks changes to the entity that is currently fetched. So if you reach into relations to show data, that view won't update when the related object changes. You can work around this by passing the object into another View marked with #ObservedObject however that doesn't work for a 1-to-many relation.
There is a neat trick using entity inheritance to allow a NSFetchedResultsController to work on multiple entities however I don't think that is suitable for this use case.
I'm pretty new to swift and core data. So if this logic doesn't make sense, please let me know.
I'm trying to save user's images to core data and be able to delete selected one.
I have Entity "Media", attribute "photo" type "binary data"
Here I convert UIImage to NSData
let data = UIImagePNGRepresentation(addedImage!) as NSData?
And save it.
let newPhoto = NSEntityDescription.insertNewObject(forEntityName: "Media", into: context)
newPhoto.setValue(imageData, forKey: "photo")
Here I try to retrieve that one selected image from core data using predicate
func deleteImage(imageData: NSData) {
let context = appDelegate.persistentContainer.viewContext
let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Media")
request.returnsObjectsAsFaults = false
request.predicate = NSPredicate(format: "photo =%#", imageData)
do {
let results = try context.fetch(request)
...
And I'm getting this error message.
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[OS_dispatch_data _bytesPtrForStore]: unrecognized selector sent to instance
Saving and getting all images from entity "Media" is working perfectly. I just can't get the specific one.
Any thoughts/suggestions?
Thanks.
Saving images like this, with external storage enabled for the property, can be OK. But there are some very unusual things going on here.
The common approach would be to have the image be part of an entity with other information, but not as the primary value of the entity. For example an entity type that represents a person and has name, address, etc, and a picture. Storing just a picture in an entity is kind of weird. You're not getting much of anything from using Core Data over something simpler. If nothing else I'd guess you had a photo ID, or title, or some other data in the entity.
The second thing relates to that-- it's extremely unusual to try to fetch using a predicate based on a binary data property. I guess it might work but only if you can ensure that the predicate value is identical to one in the persistent store all the way through-- not a single bit changed in the binary blob. That might be true but I wouldn't rely on UIKit not doing something along the way that makes it subtly different. I've never seen that particular error message but I'm not surprised that this isn't working.
Finally, it's also unusual (though not quite so strange as the above) to delete an object based on a fetch request. You would have fetched these objects before to get the images. Why fetch the same object again just to delete it? Delete the one you already fetched.
A more reliable approach would be to at least include a unique ID in the entity, which might be an integer or a UUID or whatever else works for you. Fetch this entity and display images using the photo property. When you want to delete one, take the managed object from your fetch and tell the context to delete that one. Or if fetching is necessary for some reason, use the unique ID in the predicate instead of the binary data.
You better stop doing it. Saving heavy data (image, audio, video, ...) to core data will greatly decrease fetching speed when you have more records. Instead, save image to application folder, and then only save its url to core data.
I have iOS app that takes data from the server as json and then serializes them into objects of different types. Types can be complicated, can contain subtypes, can inherit, so there is no any limitations. Another thing that makes everything even more complicated is some of types are stored as AnyObject? and only in run time they are being serialized into real types accordingly to the specific rules. Something like that:
class A {
var typeName: String?
var b: AnyObject?
}
Then when it's serialized it can be done something like that:
if let someClass = NSClassFromString(typeName) as? SomeGenericType.Type{
b = someClass.init()
}
Also querying should be done on all the data. Currently I'm trying to store all of them locally, then load into memory and query there from the code. I'm using User defaults, but they have some limitations, also I needed to provide custom coding to make it work, and each time when I add a new field it turned out that I missed something in coding and nothing works. So it's pain.
Ideally I would just do some magic command and all the objects are sent to local storage no matter how complicated they are. The same to extract them from this storage. Also, user change data so I can't just store primary Json. And I don't want to covert objects back to Jason as for it's pain too.
Any suggestions?
If you want to use sqlite then You can store whole object in one row! I means you can create table with 2 columns one is id and second is your dataobject(it's data type should be blob). Then convert your whole object into data. Then store in sqlite table and retrieve it as data then convert it to object when want to use. By this way your object will remains in same format as you asked
Firebase while meant for online synching and storage can also cache everything locally in case you are offline and perform query's against the local cache. It uses JSON.
CouchDB also has a mobile version for iOS.
Both of those are over kill if your dataset is small; you can just store it as a text file and read the JSON back in. See performance characteristics here. The graph is for a 7MB file so if you are significantly less than that your load time may be minimal.
NSKeyedArchiver.archivedData(withRootObject:) is great for storing custom objects as Data objects. The only thing you need to do to be able to use this is to make your custom objects conform to NSCoding. A great example can be found here:
Save custom objects into NSUserDefaults
Once you have the Data version of the object, it can easily be stored in UserDefaults, as a property in CoreData, or even in the app's keychain entries. Depending on your use case, sensitivity of data, and how much data you intend to store, you might want to use any number of storage methods. NSKeyedArchiver.archivedData(withRootObject:) allows you to pretty much use any of them.
Assume we have simple data model with single entity User; simple tableView_friends with fetchedResultsController_friends for show users - friends.
Assume we have search bar for searching all (not only friends) users in service, and for every typed in it character we perform search request to server, which return to us somehow filtered by character User objects. Some of this objects can already be inside local database. By app logic we don't really must save all this results in local database forever (but ok, we can, we can clear local database time to time); on other hand, if we will perform any action on some searched user, we must store this user. We want to show list of searched user in other tableView_search with fetchedResultsController_search.
Question: should I use same context for fetchedResultsController_friends and fetchedResultsController_search? If no, how can I handle situation, when I wish to edit searched user, which already exists in database and probably already local edited? If yes, how can I setup predicate for fetchedResultsController_search (server perform its own logic for search by character, which can be changed) for show exactly same result as from server?
We recently implemented a search feature in our application and had a similar issue, We had local data in core data and also remote data from our API.
You have a few options that we explored:
Save your data into core data from the API as it is retreived and
then the fetched results controller will do the rest
Manage the merge of the data yourself, you can still use NSFetchedResults controller to an extent but need to do more work
We didn't want to save all of the information returned from the API unless it was needed (the user selected it), so we come up with a simple solution that worked for our app. This may not work directly for your app, you may need a completely different solution or change some of the things we done to suit.
Firstly, To explain what we are dealing with, we had a Article entity in core data which contains around 25 properties, the API returns article objects as JSON data with the same data.
What we decided to do was to create a class which represents a simple version of an article (just enough data to show in a list view and reference it later in the API or core data) which looked something like this:
class SearchResult: NSObject {
var id:String?
var title:String?
var imageUrl:String?
var url:String?
// core data entity
init(article:Article) {
self.id = content.contentId
self.title = content.title
self.featuredImageURL = content.absoluteImagePath()
self.urlAlias = content.urlAlias
self.publishedAt = content.publishedAt
}
init(articleDictionary:NSDictionary) {
self.id = articleDictionary.objectForKeyNotNull("id") as? String
self.title = articleDictionary.objectForKeyNotNull("title") as? String
self.url = articleDictionary.objectForKeyNotNull("url") as? String
if let imageUrl = articleDictionary.objectForKeyNotNull("imageUrl") as? String {
self.imageUrl = imageUrl
}
}
}
Now using this, we can create once of these from either the core data results or from the API results. Our tableview datasource is just an array
var dataSet = [SearchResult]()
We use the NSFectchResultsController delegate methods to add/remove/re-order core data elements from the dataSet after the initial load and when we get API data we'll do something like:
dataSet = Array(Set(apiResponseArray + dataSet))
This will take an array of SearchResult items from the API, merge them with the current result set and remove duplicates. casting to a set and then back to an array will give you an array of unique results as a Set is made of unique values only.
See this reference which should help with how the delegate methods would work
I would like to separate my reference data from my user data in my Core Data model to simplify future updates of my app (and because, I plan to store the database on the cloud and there is no need to store reference data on the cloud as this is part of my application). Therefore, I've been looking for a while for a way to code a cross-store relationship using fetched properties. I have not found any example implementations of this.
I have a Core Data model using 2 configurations :
data model config 1 : UserData (entities relative to user)
data model config 2 : ReferenceData (entities relative to application itself)
I set up 2 different SQLite persistent stores for both config.
UserData config (and store) contains entity "User"
ReferenceData config (and store) contains entities "Type" and "Item".
I would like to create two single-way weak relationships as below :
A "User" has a unique "Type"
A "User" has many "Items"
Here are my questions :
How do I set up my properties?
Do I need 2 properties for each relation (one for storing Unique ID and another to access my fetched results)?
Could this weak relationship be ordered?
Could someone give me an example implementation of this?
As a follow-on to Marcus' answer:
Looking through the forums and docs, I read that I should use the URI Representation of my entity instance instead of objectID. What is the reason behind this?
// Get the URI of my object to reference
NSURL * uriObjectB [[myObjectB objectID] URIRepresentation];
Next, I wonder, how do I store my object B URI (NSURL) in my parent object A as a weak relationship? What attribute type should I use? How do I convert this? I heard about archive... ?
Then, later I should retrieve the managed object the same way (by unconvert/unarchive the URIRepresentation) and get Object from URI
// Get the Object ID from the URI
NSManagedObjectID* idObjectB = [storeCoordinator managedObjectIDForURIRepresentation:[[myManagedObject objectID] URIRepresentation]];
// Get the Managed Object for the idOjectB ...
And last but not least, shouId I declare two properties in my entity A, one for persisting of URI needs and another for retrieving direclty object B?
NSURL * uriObjectB [objectA uriObjectB];
ObjectB * myObjectB = [objectA objectB];
As you can read, I really miss some simple example to implement thes weak relationships ! I would really appreciate some help.
Splitting the data is the right answer by far. Reference data should not be synced with the cloud, especially since iCloud has soft caps on what it will allow an application to sync and store in documents.
To create soft references across to stores (they do not need to be SQLite but it is a good idea for general app performance) you will need to have some kind of unique key that can be referenced from the other side; a good old fashioned foreign key.
From there you can create a fetched property in the model to reference the entity.
While this relationship cannot be ordered directly you can create order via a sort index or if it has a logical sort then you can sort it once you retrieve the data (I use convenience methods for this that return a sorted array instead of a set).
I can build up an example but you really are on the right track. The only fun part is migration. When you detect a migration situation you will need to migrate each store independently before you build up your core data stack. It sounds tricky but it really is not that hard to accomplish.
Example
Imagine you have a UserBar entity in the user store and a RefBar entity in the reference store. The RefBar will then have a fetchedProperty "relationship" with a UserBar thereby creating a ToOne relationship.
UserBar
----------
refBarID : NSInteger
RefBar
--------
identifier : NSInteger
You can then create a fetched property on the RefBar entity in the modeler with a predicate of:
$FETCHED_PROPERTY.refBarID == identifier
Lets name that predicate "userBarFetched"
Now that will return an array so we want to add a convenience method to the RefBar
#class UserBar;
#interface RefBar : NSManagedObject
- (UserBar*)userBar;
#end
#implementation RefBar
- (UserBar*)userBar
{
NSArray *fetched = [self valueForKey:#"userBarFetched"];
return [fetched lastObject];
}
#end
To create a ToMany is the same except your convenience method would return an array and you would sort the array before returning it.
As Heath Borders mentioned, it is possible to add a sort to the NSFetchedProperty if you want but you must do it in code. Personally I have always found it wasteful and don't use that feature. It might be more useful if I could set the sort in the modeler.
Using the ObjectID
I do not recommend using the ObjectID or the URIRepresentation. The ObjectID (and therefore the URIRepresentation of that ObjectID) can and will change. Whenever you migrate a database that value will change. You are far better off creating a non-changing GUID.
The weak relationship
You only need a single value on the M side of the relationship and that stores the foreign identifier. In your object subclass you only need to implement accessors that retrieve the object (or objects).
I would go with just one store.
For storing stuff in the cloud, you will anyway have to serialize the data, either as JSON or SQL statements, or whatever scheme you prefer.
You will need a local copy of the data on the user's device, so he can access it quickly and offline. The cloud store can have only the user entity, while the local store (part of the app) can also have the reference entity.
I have a similar project with a huge reference store (20000 records) with geographic information, and user generated content ("posts"). I use a single store. When I ship the app, the "posts" entity is also defined but empty. When I update the data model I simply re-generate the whole reference store before shipping.
I see absolutely no reason to go for a cross store solution here.