I have this Backendless tables: Posts and Comments
Posts has a column comments which is has a one-to-many relationship to Comments table.
I'm try to get the comments in a latestOrder first behavior.
Currently, I have this query:
let query = BackendlessDataQuery()
let queryOptions = QueryOptions()
queryOptions.pageSize = size
queryOptions.related = ["comments", "comments.user", "user", "media"]
query.queryOptions = queryOptions
// Used PostObject since it is already mapped using `mapTableToClass`
backendlessInstance.persistenceService.of(PostObject.ofClass()).find(
query,
response: { backendlessPostsList in
let backendlessPostsListOfOffset = backendlessPostsList.getPage(offset)
guard let postObjects = backendlessPostsListOfOffset.getCurrentPage() as? [PostObject] else {
reject(BackendlessError.InvalidTypeForObject(name: "Post"))
return
}
return postObjects
},
error: { fault in
// TODO Find a way to convert a Fault to ErrorType
print("Server reported an error (1): \(fault)")
}
)
What I'm doing currently to sort Post.comments in the view model is reversing it. Post.comments.reverse().
Is there a way to explicitly sort the comments in Backendless level?
You don't have options when using related, and you should really just use it for to-one relationships, not to-many because of this. It would work ok if you have a one-to-few relationship.
So, you should make a separate request for the comments so you can specify a specific page size and sort order. Note that when you specify the variable to sort by (created) you can also add asc or desc to specify the direction of the sort (so created desc).
You could use the sort function.
let sortedComments = Post.comments.sort { $0.date > $1.date }
Of corse your comments will need a published date field for this to work.
Related
When retrieving all reviews for a movie (one-to-many) relationship, which snippet of code would you use and why?
static func getReviewsByMovieId(movieId: NSManagedObjectID) -> [Review] {
// OPTION 1
// SQL call to get movie
guard let movie = CoreDataManager.shared.getMovieById(id: movieId),
// SQL call to get reviews for the movie
let reviews = movie.reviews
else {
return []
}
return (reviews.allObjects as? [Review]) ?? []
// OPTION 2
// SQL call to get reviews for a particular movie
let request: NSFetchRequest<Review> = Review.fetchRequest()
request.predicate = NSPredicate(format: "movie = %#", movieId)
do {
return try CoreDataManager.shared.viewContext.fetch(request)
} catch {
return []
}
}
Personally I'd do something more like the first, but it's mostly a matter of style. It's very unlikely to matter to performance unless you're doing this a lot. In that case I'd want to try both and profile the results.
I don't know how your getMovieById(id:) works, but if it's using a fetch request I'd suggest replacing it with either object(with:) or existingObject(with:). Both simply look up an object by ID without needing a predicate. The first is probably slightly faster, and the second is safer, so the choice depends on how certain you can be that the object ID is valid.
I am performing a core data migration. I have this subclass for my migration:
class TagtoSkillMigrationPolicyV1toV2: NSEntityMigrationPolicy {
override func createDestinationInstances( forSource sInstance: NSManagedObject, in mapping: NSEntityMapping, manager: NSMigrationManager) throws {
// 1 - create destination instance of skill and skillmetadata
let skillEntityDescription = NSEntityDescription.entity( forEntityName: "Skill",in: manager.destinationContext)
let newSkill = Skill(entity: skillEntityDescription!, insertInto: manager.destinationContext)
let skillMetaDataDescription = NSEntityDescription.entity(forEntityName: "SkillMetaData", in: manager.destinationContext)
newSkill.metaData = SkillMetaData(entity: skillMetaDataDescription!, insertInto: manager.destinationContext)
var posts = sInstance.value(forKey: "posts")
posts.sort {
($0.timeStamp! as Date) < ($1.timeStamp! as Date)
}
if let mostRecentThumbnail = posts.first?.postImage {
let thumbnail = Utils.makeThusmbnail(url: URL(string: mostRecentThumbnail)!, for: 150.0)
newSkill.metaData?.thumbnail = thumbnail?.pngData()
}
if let name = sInstance.value(forKey: "name"){
newSkill.setValue(name, forKey: "name")
}
manager.associate(sourceInstance: sInstance, withDestinationInstance: newSkill, for: mapping)
}
}
This custom migration is from my old Tag entity to my new Skill entity. For each Tag, it creates a skill entity and a skillmetaData entity that it attaches to it. There are no entries in the .xcmappingmodel file, as I am doing everything manually - please let me know if I still need entries there.
Right now, when it runs, it gives a signal SIGABRT:
Could not cast value of type '_NSFaultingMutableOrderedSet' (0x7fff87c44e80) to 'NSArray' (0x7fff87c51fb0).
2020-04-14 15:55:40.108927-0700 SweatNetOffline[28046:3645007] Could not cast value of type '_NSFaultingMutableOrderedSet' (0x7fff87c44e80) to 'NSArray' (0x7fff87c51fb0).
When I set a breakpoint and inspect sInstance.value(forKey: "posts"), I get
▿ Optional<Any>
- some : Relationship 'posts' fault on managed object (0x6000010dcf00) <NSManagedObject: 0x6000010dcf00> (entity: Tag; id: 0xdf4b05c2e82b2c4e <x-coredata://A6586C18-60C3-40E7-B469-293A50EB5728/Tag/p1>; data: {
latestUpdate = "2011-03-13 00:17:25 +0000";
name = flowers;
posts = "<relationship fault: 0x600002607f40 'posts'>";
uuid = "4509C1B3-7D0F-4BBD-AA0B-7C7BC848DA80";
})
so its not entirely loading those posts - its giving that fault there.
The goal is to get the thumbnail from the latest of those posts. How can I access this relationship item?
I found a way to do it using
let lastThumbnail = sInstance.mutableOrderedSetValue(forKeyPath: "posts.postImage").lastObject
Reading this helped a lot. My understanding is at this point in the migration, we are dealing with NSManagedObjects and never the actual Model which would have attributes like "posts" that it usually does in core data.
In Objective-C especially and swift to some extent, NSObjects are accessed with key-value coding. They are accessed indirectly - meaning instead of reading the object itself, you are reading what the NSMutableOrderedSet wants to show you for that key - so it can do stuff to it before presenting it. Thus you can't for instance, fetch just "forKey: posts" and then expect posts to have the attributes your post usually does. Anyone is welcome to flesh out this explanation if they know more :).
One thing I was trying to do is to fetch by that key-path in a sortable way - so that I can find the post that has the most recent timestamp and then use that associated thumbnail. Instead I am using .lastObject which works because this is an NSMutableOrderedSet and not an NSMutableSet because my original model specified this relationship to be an ordered relationship. This means that it will pick the last inserted item which is ok for me for now. Still interested in a way to filter that result though if anyone has advice.
My app has data that looks like this.
class ShelfCollection: Object {
let shelves: List<Shelf>
}
class Shelf: Object {
let items: List<Item>
}
class Item: Object {
var name: String
let infos: List<String>
}
I'm trying to get all shelves in a shelf collection where any items match the query either by name or by an element in their infos list. From my understanding this predicate should be correct, but it crashes.
let wildQuery = "*" + query + "*"
shelfResults = shelfCollection.shelves.filter(
"SUBQUERY(items, $item, $item.name LIKE[c] %# OR SUBQUERY($item.infos, $info, info LIKE[c] %#).#count > 0).#count > 0",
wildQuery, wildQuery
)
It complies as a NSPredicate, but crashes when Realm is attempting to parse it, throwing me
'RLMException', reason: 'Object type '(null)' not managed by the Realm'
I suspect the nested subquery might be what fails, but I don't know enough about NSPredicate to be sure. Is this an acceptable query, and how can I make it.. work?
This is an answer and a solution but there's going to be a number of issues with the way the objects are structured which may cause other problems. It was difficult to create a matching dataset since many objects appear within other objects.
The issue:
Realm cannot currently filter on a List of primitives
EDIT: Release 10.7 added support for filters/queries as well as aggregate functions on primitives so the below info is no longer completely valid. However, it's still something to be aware of.
so this Item property will not work for filtering:
let infos: List<String>
However, you can create another object that has a String property and filter on that object
class InfoClass: Object {
#objc dynamic var info_name = ""
}
and then the Item class looks like this
class Item: Object {
var name: String
let infos = List<InfoClass>()
}
and then you filter based on the InfoClass object, not it's string property. So you would have some objects
let info0 = InfoClass()
info0.info_name = "Info 0 name"
let info1 = InfoClass()
info1.info_name = "Info 1 name"
let info2 = InfoClass()
info2.info_name = "Info 2 name"
which are stored in the Item->infos list. Then the question
I'm trying to get all shelves in a shelf collection...
states you want to filter for a collection, c0 in this case, shelves whose items contain a particular info in their list. Lets say we want to get those shelves whose items have info2 in their list
//first get the info2 object that we want to filter for
guard let info2 = realm.objects(InfoClass.self).filter("info_name == 'Info 2 name'").first else {
print("info2 not found")
return
}
print("info2 found, continuing")
//get the c0 collection that we want to get the shelves for
if let c0 = realm.objects(ShelfCollection.self).filter("collection_name == 'c0'").first {
let shelfResults = c0.shelves.filter("ANY items.infoList == %#", info2)
for shelf in shelfResults {
print(shelf.shelf_name)
}
} else {
print("c0 not found")
}
I omitted filtering for the name property since you know how to do that already.
The issue here is the infos could appear in many items, and those items could appear in many shelf lists. So because of the depth of data, with my test data, it was hard to have the filter return discreet data - it would probably make more sense (to me) if I had example data to work with.
Either way, the answer works for this use case, but I am thinking another structure may be better but I don't know the full use case so hard to suggest that.
I am using Core Data to fetch the list of clinics, the clinics has relations named doctors mapped to Doctor entity.
My issue is, every time I fetch doctors, which is of type NSSSet, the record is fetched randomly, I want to sort it alphabetically using Doctor.name
I tried the following
self.doctorList = clinic.doctors?.sortedArray(using: [NSSortDescriptor(key: "name", ascending: true)])
Where am I going wrong?
Thanks.
(NS)Set is an unordered collection type.
To order a Set convert it to an array with the allObjects property and sort the array.
self.doctorList = (clinic.doctors!.allObjects as! [Doctor]).sorted(by: { $0.name < $1.name })
You can even sort the set directly but the result is always an array
self.doctorList = (clinic.doctors as! Set<Doctor>).sorted(by: { $0.name < $1.name })
And if the set is declared as native Set<Doctor> anyway you can omit the type cast.
I recommend to declare the doctors relationship as non-optional. If there is no associated doctor the set is empty. But a clinic without any doctor is very unlikely ;-)
Quick background: I have en ExamEntity, which is linked via a relationship to sections covered in the exam (SectionEntity), which is linked via a relationship to the subsections under each section (SubsectionEntity).
What I'm try to do is fetch the sections and related subsections which will be covered be a selected Exam, and store them in a dictionary which can hold multiple string values per key.
My code starts like this:
var sectionsDictionary = [String:[String]]()
//---Fetch Section Entity
let sectionPredicate = NSPredicate(format: "(exam = %#)", selectedExam)
let sectionsFetchRequest:NSFetchRequest = SectionEntity.MR_requestAllWithPredicate(sectionPredicate)
let fetchedSections:NSArray = SectionEntity.MR_executeFetchRequest(sectionsFetchRequest)
All good here. But then I try and fetch the subsections related to the selected sections and things go wrong.
following Code:
for section in fetchedSections {
let sectionName = section.valueForKey("name") as! String
// //error occurs on the line below
let subsectionPredicate = NSPredicate(format: "(section = %#)", section as! SectionEntity)
let subsectionsFetchRequest:NSFetchRequest = SubsectionEntity.MR_requestAllWithPredicate(subsectionPredicate)
let fetchedSubsections:NSArray = SubsectionEntity.MR_executeFetchRequest(subsectionsFetchRequest)
The error I get is:
Could not cast value of type 'NSManagedObject_SectionEntity_' (0x7fb363e18580) to 'MyApp.SectionEntity' (0x10c9213e0).
The next few lines are supposed to iterate through the fetched results, adding their names as values to the dictionary with section names as keys.
for subsection in fetchedSubsections{
let subsectionName = subsection.valueForKey("name") as! String
sectionsDictionary[sectionName] = [subsectionName]
}
}
thanks in advance.
You're not defining the type of the class for your record
Go to the Project.xcdatamodeld file, and select your SectionEntity
Check if the field Class is like this
If it is then write in the Class field "SectionEntity" and press Enter, it should be like this
Then try again
You need to make sure that the elements of the collection you enumerate are of the correct type.
Presumably, Magical Record will return an NSArray of NSManagedObject items. You need to make sure Swift knows what type section is.
for object in fetchedSections {
let section = object as! SectionEntity
// ...
}
Also, see my answer about the unsuccessful casts to NSManagedObject subclasses.