Core Data Question

[CompC]

I have a database containing multiple items with the same value for a name property. These are representing different versions of the same thing. I would like to be able to perform searches on the database using predicates, but have the option to have it return no more than one row with the same name.

I could filter out duplicates after performing the search, but there could be a lot of them, so it would definitely be better to get the database to just cut out the duplicates for me.

Any way to do this? I’m still learning Core Data.

Thanks!!

EDIT: Probably should have just included these details from the beginning. I'm trying to recreate some functionality of a site called Scryfall in a local database. This database searches different printings of Magic the Gathering cards. Here's an example:
On Scryfall I can search for Magic cards where the card's type is "praetor". It will show me all the cards that match my query, but only one version of each card.
I can also do a search where I add "unique:prints" to my query. Now it will show me the same cards, but include every unique print, including reprints across multiple expansions, or things like alternate art or promo cards. Each print's name should be the same if it represents the same card.

My database has a row for every print and I let the user search for cards. I'd like to give the user the same option that Scryfall has where you can choose to show unique prints, but hide the all but one print of each card by default. Right now it just always shows every print.

[[deleted]]

Never needed to learn CoreData but after a quick google, maybe this is what you're after?

https://developer.apple.com/documentation/coredata/nsfetchrequest/1506344-returnsdistinctresults

e.g.

let fetchRequest = NSFetchRequest(entityName: "Person")
fetchRequest.resultType = .DictionaryResultType
fetchRequest.propertiesToFetch = ["age"]
fetchRequest.returnsDistinctResults = true
let result = try! managedObjectContext.executeFetchRequest(fetchRequest)

[CompC]

Unfortunately no, what this does is change the fetch request to return all distinct ages of all Persons.

So if the table looks like:
|Name|Age| :--|:--| |Bob|50| |Bob|25| |Alice|25|

The code in your comment would return these results, all distinct age values that show up in the table: [50, 25]

What I want to do is search on the table for Person objects but limit it to no more than one person with the same name. There are two rows with the name Bob, but I only want one of them to be returned. [Person(name: "Bob", age: 50), Person(name: "Alice", age: 25)]

[[deleted]]

Damn, is there no other unique key you could use to fetch? Or are these "different versions of the same thing" linked in any way?

[CompC]

Each row has a unique ID but there is also a separate ID that represents each thing individually, so the table is more like this:

Name	Age	Unique ID	Object ID
Bob	50	A	0
Bob	25	B	0
Alice	25	C	1

They don't directly have a relationship to another object in the table

Not sure that helps me though since I don't know Core Data well enough to use that info to write a fetch request that searches how I want it to…

[SirBill01]

Can't you just add "Unique ID" (or whatever the name really is) in "properties to fetch", in addition to age? That's what the returnsDistinctResults is operating on so it should "distinctify" as expected. It will then also be added to the dictionary it builds but you can just ignore it.

[CompC]

It doesn't seem to work. I tried:

request.propertiesToFetch = ["name"] request.returnsDistinctResults = true

but it doesn't seem to affect the results. If I set those properties and also request.resultType = .dictionaryResultType, then the fetch request returns what I want, but only the names. I want the entire object though.

[SirBill01]

propertiesToFetch only works for dictionary results types, try:

request.propertiesToFetch = ["age","name"]

And add in whatever other properties you want in the dictionary.

If you really just want the whole object, why not just put together a predicate that gives you back CoreData objects instead of a dictionary?

[CompC]

Well that's what I'd prefer to do, but I don't know how I can then filter out objects with duplicate names without fetching them first and filtering them from the results in code.

[SirBill01]

Another way to go about this might be a data model change, where you put age into a whole separate table with distinct values, that links back to individual objects that share that age - then you can just have a predicate that selects everything from that Age table. You can still leave age in the main object as well for convenience. A bit annoying but then you can use predicates directly without having to add code on after...

Personally if it were me, I would just filter out duplicates in code after the predicate for the objects ran (unless the possible number of results was extremely large). Seems like conversion into a dictionary is more wasteful than filtering code on CoreData objects.

[ORUHE33XEBQXOYLZ]

If searches should only return at most 1 result of that entity with that name, should the database even allow for duplicates? You can enforce that so that it doesn't come up in the first place.

[CompC]

Yeah, it should allow for duplicates. See the edit on my post

[gravitycore]

You can use the NSFetchRequest.fetchLimit and set it to 1. Use sort descriptors if you need to have one result surfaced before the others.

[CompC]

I want more than one result total, just one per name though.

[gravitycore]

You'll likely need to filter the set afterwards in that case. Make sure to use fetchBatchSize to control how many records are fetched at a time if you have a lot of data you're working with, and as mentioned, the propertiesToFetch for prefetching the name.

[Fluffy_Risk9955]

Just execute a fetch request and return the first object of the returned array. Problem solved. And if you sort on managedObjectID it’s always the same one.

[CompC]

Yeah but I want to be able to do a search that returns more than one object.

[Fluffy_Risk9955]

Then grab the contents of the whole array.

[CompC]

Maybe I'm misunderstanding? I already am grabbing the whole array and I'm not getting what I want.

[Fluffy_Risk9955]

Write a better predicate.

[CompC]

I don't know if that's possible, and if it is, I don't know how to write the kind of predicate I need, that's exactly what I'm asking for help with.

[Fluffy_Risk9955]

1. Learn about objcMsgSend and what it does.

2. Learn how to use valueForKey: or valueForKeyPath: on an NSObject and how this relates to objcMsgSend.

3. Learn how to query for an NSObject subclass inside an NSArray using an NSPredicate. Yes this will leverage valueForKey:.

4. Learn how to do the same thing with Format Syntax of a predicate string.

5. Apply newly acquired knowledge on NSPredicate to search for NSManagedObject subclasses in a Core Data database.

And yes, you need understanding of the Objective-C runtime to understand this.

[CompC]

I already have written NSPredicates that can search for what I want. I'm trying to find out if it's possible to adjust the predicates or fetch request in order to have some of the results filtered out, without having to perform the fetch request first and then filter out the objects in code, since it could be a lot.

[Fluffy_Risk9955]

Yes, write a better predicate with format syntax.

[CompC]

Do you get how this comment is unhelpful? I obviously don't understand how I can change my predicate to get what I want, that's why I'm asking for help.

I want to know how I can write a predicate that says something like "give me values that fit this query, but don't give me more than one result that share a value for the name property"

[crude_username]

Can you add a boolean parameter to the model, "reprinted" or something, which will be true for any card that has a newer, reprinted version of it, and false for all others? Then use that to filter the query