iam a swift beginner, i wanna code a little app for me and i have a problem with it. My app has three entities (Templates, Records and Positions), here you can see:
Datamodel
At one part of the app i can add new Records and for that Record i add some Positions (xpos and ypos).
At a part I have a tableview were i list my Records. Now i wanna click one Record in the table and i want to get all Position-Attributes linked with the particular selected Record.
With that code i can get all xpos-Positions but how can i get a single xpos? :)
guard let moc = self.managedContext else {
return
}
let fetchRequest: NSFetchRequest<Records> = Records.fetchRequest()
do {
let searchResults = try moc.fetch(fetchRequest)
let xpos = searchResults[0].positions?.mutableArrayValue(forKey: "xpos")
print(xpos)
} catch {
print("Error with request: \(error)")
}
You want to look into using NSPredicate. NSPredicate basically lets you define the conditions which the record needs to meet to be included in the results. Think of it as a filter if you will.
fetchRequest.predicate = NSPredicate(format: "uniqueID = %#", arugments:...)
Related
I have close to 7K items stored in a relation called Verse. I have another relation called Translation that needs to load 7K related items with a single call from a JSON file.
Here is my code:
let container = getContainer()
container.performBackgroundTask() { (context) in
autoreleasepool {
for row in translations{
let t = Translation(context: context)
t.text = (row["text"]! as? String)!
t.lang = (row["lang"]! as? String)!
t.contentType = "Verse"
t.verse = VerseDao.findById(row["verse_id"] as! Int16, context: context)
// this needs to make a call to the database to retrieve the approparite Verse instance.
}
}
do {
try context.save()
} catch {
fatalError("Failure to save context: \(error)")
}
context.reset()
}
Code for the findById method.
static func findById(_ id: Int16, context: NSManagedObjectContext) -> Verse{
let fetchRequest: NSFetchRequest<Verse>
fetchRequest = Verse.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "verseId == %#", id)
fetchRequest.includesPropertyValues = false
fetchRequest.fetchLimit = 1
do {
let results =
try context.fetch(fetchRequest)
return results[0]
} catch let error as NSError {
print("Could not fetch \(error), \(error.userInfo)")
return Verse()
}
}
This works fine until I add the VerseDao.findById, which makes the whole process really slow because it has to make a request for each object to the Coredata database.
I did everything I could by limiting the number of fetched properties and using NSFetchedResultsController for data fetching but no luck.
I wonder if there's any way to insert child records in a more efficient way? Thanks.
Assuming your store type is persistent store type is sqlite (NSSQLiteStoreType):
The first thing you should check is whether you have an Core Data fetch index on the Verse objects verseId property. See this stack overflow answer for some introductory links on fetch indexes.
Without that, the fetch in your VerseDao.findById function may be scanning the whole database table every time.
To see if your index is working properly you may inspect the SQL queries generated by adding -com.apple.CoreData.SQLDebug 1 to the launch arguments in your Xcode scheme.
Other improvements:
Use NSManagedObjectContext.fetch or NSFetchRequest.execute (equivalent) instead of NSFetchedResultsController. The NSFetchedResultsController is typically used to bind results to a UI. In this case using it just adds overhead.
Don't set fetchRequest.propertiesToFetch, instead set fetchRequest.includesPropertyValues = false. This will avoid fetching the Verse object property values which you don't need to establish the relation to the Translation object.
Don't specify a sortDescriptor on the fetch request, this just complicates the query
hi i'm not really understanding how the fetch filter works can anyone help me please? So i currently have this as my fetchall function which displays all of my items within my entity
im having trouble of filtering only one attribute which is a boolean. I want it to only display attributes that are true.
thankyou!
func fetchAllItems(){
let request = NSFetchRequest<NSFetchRequestResult>(entityName: "AllItems")
do{
let result = try managedObjectContext.fetch(request)
beastList = result as! [AllItems]
} catch { print("\(error)")
}
}
Code:
let request = NSFetchRequest<NSFetchRequestResult>(entityName: "AllItems")
request.predicate = NSPredicate(format: "something = %#", argumentArray: [true])
Note: Replace something with your boolean field name
Better way to create request
let request : NSFetchRequest<AllItems> = AllItems.fetchRequest()
Naming convention:
It is better to name your entity in singular form, so that each row in your entity could be in singular form.
AllItems seems very generic, if you are storing cars, the entity name could be Car. The variable that stores the result of the fetch request could be cars.
Reference:
https://developer.apple.com/documentation/foundation/nspredicate
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Predicates/Articles/pSyntax.html
I'm working on a word game and have bundled a complete list of english words using a sqlite database. I'm trying to find the best way to search the database for a given string to determine if it's a word.
At this point I can get the whole database out into an array:
func fetchWords() {
if let managedObjectContext = (UIApplication.shared.delegate as? AppDelegate)?.managedObjectContext {
let wordsFetch = NSFetchRequest<NSFetchRequestResult>(entityName: "EnglishWord")
do {
englishWords = try managedObjectContext.fetch(wordsFetch) as! [EnglishWord]
print(englishWords[123323].word)
//Prints "injustices"
} catch {
//error handling
}
}
}
Now. What I really want to do is pass in a given string and see if it exists as a word in my database. I have a clunky solution with predicates, e.g:
func fetchWordsToArray() {
if let managedObjectContext = (UIApplication.shared.delegate as? AppDelegate)?.managedObjectContext {
let wordsFetch = NSFetchRequest<NSFetchRequestResult>(entityName: "EnglishWord")
let searchWord = ["dastardly"]
let searchPredicate = NSPredicate(format: "word = %#", argumentArray: searchWord)
do {
englishWords = try managedObjectContext.fetch(wordsFetch) as! [EnglishWord]
let resultFilteredByPredicate = (englishWords as NSArray).filtered(using: predicate)
print(resultFilteredByPredicate)
} catch {
//error handling
}
}
}
But in order to use the filtered function I have to convert to an NSArray which means I can't work with the results directly (e.g. get resultFilteredByPredicate.word).
Also it feels like I'm probably going about this all the wrong way. Since everything has to go into an array first, I must be losing a lot of the value of using an sqlite database in the first place.
Any suggested approaches for better working with the database?
Many thanks in advance for any help!
To make the database do the filtering (which could then be optimized automatically with an index), put a predicate on the original fetch request:
let formatRequest : NSFetchRequest<Word> = Word.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "word == %#", searchWord)
let fetchedResults = try context.fetch(fetchRequest) as! [Word]
But Core Data is a framework to manage your objects.
If you decide that your words are not objects but just values, you could replace Core Data with some other library that does SQL directly, and execute an SQL query instead:
SELECT * FROM EnglishWord WHERE word = ?
In my iOS Xcode8 project using Swift, I'm performing a fetch of my Core-Data:
func searchFoods() {
let context: NSManagedObjectContext = appDel.managedObjectContext
let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Foods")
print("Searching Database for \(searchVariable)...")
var subPredicates : [NSPredicate] = []
let codeSearch = NSPredicate(format: "codeText contains[c] %#", "\(searchVariable)")
subPredicates.append(codeSearch)
request.predicate = NSCompoundPredicate(orPredicateWithSubpredicates: subPredicates)
request.returnsObjectsAsFaults = false
do {
let results = try context.fetch(request)
if results.count > 0 {
for result in results as! [NSManagedObject] {
if let item = result.value(forKey: "title") as? String {
// Maybe put a loop of some kind to only append the found item count??
searchArray.append(item)
myTableView.reloadData()
}
}
}
} catch {
print("Fetch failed...")
}
}
However my searchArray that is a [String] of the search results created many duplicates I know are not there; it's listing them 2 or 3 times. Can't figure out how to limit the appending to just the result count amount. If I search fruit, it might return an array like bananas, strawberries, peaches, oranges, bananas, strawberries, peaches, oranges etc, repeating. Can someone please help?
There's nothing in the code that would cause duplicates in the fetch results. If they're not actually present in the persistent store, the likely cause is that you're doing this:
searchArray.append(item)
But there's no sign of ever clearing out past results. Your sample results are consistent with this-- if there are four results, you add them to the array once, then later add them again.
It's also possible that there's a problem with your table view data source methods, but you're probably driving that directly from the contents of searchArray.
i would like to request all my core data where the "mhd" field >= the date of today. this code works fine:
func DatenAbrufen() {
let calendar = NSCalendar.currentCalendar()
let AktuellesDatum = calendar.startOfDayForDate(NSDate())
let fetchRequest = NSFetchRequest(entityName: "LM_ITEMS")
fetchRequest.predicate = NSPredicate(format: "mhd >= %#", AktuellesDatum)
if let fetchResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: nil) as? [LM_ITEMS] {
lebensmittel = fetchResults
}
}
how can i delete in this query all other data, where "mhd" is not >= date of today?
If you need to support iOS 8.x, or OS X Yosemite, or older, you'll need to fetch each object and then delete it. Write another predicate that is the opposite of the one you have. Execute that fetch request. Then iterate over the found objects and send each one a deleteObject. Save your managed object context. This can take quite a while, if you're deleting lots of objects.
For iOS 9 and El Capitan, you can use NSBatchDeleteRequest, described in WWDC 2015 session 220. For large numbers of deleted objects, NSBatchDeleteRequest is much faster. You're not loading each object into memory just to delete it. You also bypass some validation rules, and must refresh your MOC for it to see changes–which might be undesired consequences, depending on your usage.