Looking for help constructing a query.

[Kunal Sood]

My models look like this: 

@interface FeedItem : RLMObject
@property NSString *title;
@property NSString *uniqueIdentifier;
@property BOOL isUnread;
@property Feed *feed;
@end


RLM_ARRAY_TYPE(FeedItem)
RLM_ARRAY_TYPE(Folder)
@interface Feed : RLMObject
@property NSString *title;
@property NSString *uniqueIdentifier;
@property RLMArray<FeedItem *><FeedItem> *items;
@property RLMArray<Folder *><Folder> *folders;
@end


RLM_ARRAY_TYPE(Feed)
@interface Folder : RLMObject
@property NSString *title;
@property NSString *uniqueIdentifier;
@property RLMArray<Feed *><Feed> *feeds;
@end

All relationships are set explicitly (i.e. I do not use backlinks), and before adding a Folder to a Feed's 'folders' property, or, before adding a Feed to a Folder's 'feeds' property, I check if one already exists to avoid duplication.

Now, I have a Folder, and need to fetch all FeedItem objects that are related to all Feed objects related to this Folder in this folder. I have tried every kind of predicate format I could think of..

From a simple:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@ IN feed.folders", folder];
RLMResults *feedItems = [FeedItem objectsWithPredicate:predicate];

which fails with exception 'Invalid predicate', reason: 'Predicate with IN operator must compare a KeyPath with an aggregate'.

to a SUBQUERY like:

NSString *folderUniqueIdentifier = folder.uniqueIdentifier;

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SUBQUERY(feed.folders, $folder, $folder.uniqueIdentifier = %@).@count > 0", folderUniqueIdentifier];

RLMResults *feedItems = [FeedItem objectsWithPredicate:predicate];

which fails with exception 'Invalid predicate', reason: 'Aggregate operations can only be used on RLMArray properties'.

The only one I have been able to get to work is:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"feed IN %@", folder.feeds];

RLMResults *feedItems = [FeedItem objectsWithPredicate:predicate];

But, this one also does not seem to auto-update when feeds are added to, or removed from the Folder's 'feeds' property. (I end up having to add a KVO observer to the Folder's 'feeds' property and execute the query again when said observer notifies of a change).

So, any help with constructing a query for this that can auto-update would be greatly appreciated.

[Kunal Sood]

Thanks Mark,

I notice that #3216 got merged into master, so gave it a shot, and it does exactly what what I was after.

Thanks again!

Best,

Kunal

On 16 Feb 2016, 3:56 AM +0530, Mark Rowe <he...@realm.io>, wrote:

Hi Kunal,

You mentioned three attempts at queries to retrieve FeedItem objects related to a given Folder:

1. "%@ IN feed.folders". As you've seen, this form of expression is not currently supported by Realm. I've added it to the list on our Unsupported predicates GitHub issue.
2. "SUBQUERY(feed.folders, …).@count > 0". We are currently erroneously rejecting this query. I've submitted a pull request to address this.
3. "feed IN %@". As you've seen, this expression operates on a snapshot of the substituted RLMArray rather than the live contents as expected. I've filed a GitHub issue about this.

You'll need to stick with option 3 for now, but when #3216 ships you can switch to option 2 to avoid explicitly recreating the query in response to notifications.

--
Mark Rowe

{#HS:171302366-3042#}

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