CODEWITHSUNDEEP
iosobjective-ccore-data
coder
coder

Reputation: 10530

Core data NSPredicate with multiple relationships

I need to make a fetch request, and I can't quite figure out the NSPredicate I need for the request. Here's the scenario:

I have a List entity, and the list object has a to-many relationship to a set of Request objects. Each Request object has a to-many relationship to a set of Tags objects.

I need perform a fetch request on the Tags entity, but I can't figure the NSPredicate for the call. I would like to only pull out the tags that are in all the tags of all the requests of a specific list.

Here's what I've tried:

 NSPredicate *predicate = [NSPredicate predicateWithFormat:@"tag in ANY list.requests.tags"];

But I know this isn't right. I'm not able to specify my list, also list.requests returns an NSSet, and I can't just do .tags on a set. I'm pretty stuck.

I've attached a pic of relationships in case that helps:

enter image description here

Upvotes: 2

Views: 922

Answers (1)

Léo Natan
Léo Natan

Reputation: 57060

Create a sub-query.

NSPredicate* predicate = [NSPredicate predicateWithFormat:@"SUBQUERY(requests, $request, $request.list = %@).@count > 0", list];

This counts the requests which have a specified list; if the count is more than one, the tag is taken.

You can also do:

[list.requests valueForKeyPath:@"tags.@distinctUnionOfSets"]

This uses key-value coding to retrieve all the tags that belong to all the requests and unifies them into a set. This method is less efficient than the former.

Upvotes: 2

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