Realm-cocoa: Support queries on inverse relationships (backlinks)

[joshuadutton]

It looks like the only way I can query inverse relationships is to create explicit links instead of using the recommendation for creating inverse relationships found in the docs (http://realm.io/docs/cocoa/#relationships).

Note: creating explicit links is explained here: https://github.com/realm/realm-cocoa/issues/677#issuecomment-50182237

[jpsim]

Thanks for filing this, @joshuadutton. We agree that it would be nice to expose a more direct way to do this through property-backed inverse relationships, though it is possible to do many queries involving inverse relationships today.

If you have a specific query you'd like to make, you can post it here and we might be able to find a way to do that using our current API.

[joshuadutton]

I have an Asset class that has a to-many relationship with a Tag class so that an asset can have multiple tags. My Tag class has an inverse relationship so I can see which assets have been tagged with a given tag.

Given a bunch of assets, I want a query that will return all the tags they collectively have. I assumed this would work:

[Tag objectsWhere:@"ANY assets IN %@", assets]

This throws an error with the way I have implemented the inverse relationship since the assets property is not stored in the database.

Edit: The above query works with an explicit link.

[Kirow]

I have a problem related to this issue

@interface Big : RLMObject
@property(readonly) NSArray *smallArray;
@end
@implementation Big
- (NSArray *)smallArray {
    return [self linkingObjectsOfClass:@"Small" forProperty:@"big"];
}
@end

@interface Small : RLMObject
@property Big *big;
@end

Need to delete all Big where smallArray.count == 0.
I made it this way:

RLMResults *bigs = [Big allObjectsInRealm:realm];
    for (Big *item in bigs) {
        if (item. smallArray.count == 0) {
            [realm transactionWithBlock:^{
                [realm deleteObject:item];
            }];
        }
    }

Any way to make it like [realm deleteObjects:[Big allObjectsInRealm:realm where:@"smallArray.count = 0"]]?

[alazier]

@Kirow count queries are not yet supported. There is an open issue for it here #1166

[jpsim]

count queries are not yet supported

Even if they were, it's not possible to include a property that isn't stored in Realm in a predicate (e.g. smallArray in this case).

What necessary for the time being is to formulate a predicate based on the results of calling linkingObjectsOfClass:forProperty:.

[pizthewiz]

I just ran into this as well and as @joshuadutton mentioned, (until this is supported) it can be worked around via explicit relationship.

[bcapps]

Just wanted to chime and say that this would be great. We recently ran into an issue where we tried to filter on an inverse relationship and stumbled upon this. I see that the issue blocked, but simply wanted to state how nice it would be to be able to use inverse relationships to efficiently filter results. Thanks!

[segiddins]

@bcapps there's actually an open PR that adds this. The blocked tag can probably be removed now.

[siuying]

Suppose we have model like this:

class Car : Object {
   var owner: Person? {
        return linkingObjects(Person.self, forProperty: "cars").first
    }
}

It would be great if we have simple way to find all cars with no owner, like realm.objects(Car).filter("owner is NULL")

[jpsim]

@siuying yes, that's part of what this issue is tracking.

[oscarvgg]

+1 for this feature

[jrmybrault]

+1, any updates on this ? In my opinion one of the most frustrating thing about Realm right now.

[jpsim]

+1, any updates on this ? In my opinion one of the most frustrating thing about Realm right now.

Agreed. We're actively working on it.

[tenhow]

+1 for this feature

[desmond0321]

• 1 for this feature as well.

[wolfAle]

Another +1

[juliensagot]

Any updates ?

[maxoly]

+1

[michaelloistl]

Any update on that one?

[bdash]

I started working on this earlier this week. I'll update this issue when I have something to share.

[Farmertjes]

+1

[timothyrobb]

+1, looking forward to seeing your solution for this

[listepo]

+1

[bdash]

See #3419 to get a preview of the API for this.

[jkmathew]

Hi @bdash
I have related issue.
I have Friend class and a FriendGroup class. FriendGroup has a to may relation to Friend also, Friend has an inverse relation to FriendGroup. Now I want to fetch all friends to a Results<Friend> who are not added to any groups.

My class declaration is like,

class Friend: Object {
    @objc dynamic var name = ""
    let group = LinkingObjects(fromType: FriendGroup.self, property: "members")    
}

class FriendGroup: Object {
    @objc dynamic var name = ""

    let members = List<Friend>()
}

And I tried fetching like

let predicae = NSPredicate(format: "group.count < %ld", 1)
friends = realm?.objects(Friend.self).filter(predicae).sorted(byKeyPath: "name")

But its crashing with stack trace

*** Terminating app due to uncaught exception 'Invalid property name', reason: 'Property 'count' not found in object of type 'FriendGroup''
*** First throw call stack:
(
    0   CoreFoundation                      0x0000000106ca51cb __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x0000000105a2bf41 objc_exception_throw + 48
    2   Realm                               0x000000010465e4c1 _ZL15RLMPreconditionbP8NSStringS0_z + 657
    3   Realm                               0x0000000104666b97 _ZN12_GLOBAL__N_120key_path_from_stringEP9RLMSchemaP15RLMObjectSchemaP8NSString + 1015
    4   Realm                               0x0000000104663f8c _ZN12_GLOBAL__N_112QueryBuilder30column_reference_from_key_pathEP15RLMObjectSchemaP8NSStringb + 140
    5   Realm                               0x0000000104662fa2 _ZN12_GLOBAL__N_112QueryBuilder22apply_value_expressionEP15RLMObjectSchemaP8NSStringP11objc_objectP21NSComparisonPredicate + 338
    6   Realm                               0x0000000104660033 _ZN12_GLOBAL__N_112QueryBuilder15apply_predicateEP11NSPredicateP15RLMObjectSchema + 5075
    7   Realm                               0x000000010465e67b _Z19RLMPredicateToQueryP11NSPredicateP15RLMObjectSchemaP9RLMSchemaRN5realm5GroupE + 347
    8   Realm                               0x000000010479b57b _ZZ35-[RLMResults objectsWithPredicate:]ENK4$_11clEv + 283
    9   Realm                               0x0000000104795206 _ZL15translateErrorsIZ35-[RLMResults objectsWithPredicate:]E4$_11EDaOT_P8NSString + 38
    10  Realm                               0x0000000104795188 -[RLMResults objectsWithPredicate:] + 72
    11  RealmSwift                          0x00000001050e0459 _T010RealmSwift7ResultsC6filterACyxGSo11NSPredicateCF + 153
    12  Invite                              0x00000001041a4212 _T06Invite20ImportViewControllerC11viewDidLoadyyF + 514
    13  Invite                              0x00000001041a4a94 _T06Invite20ImportViewControllerC11viewDidLoadyyFTo + 36
    14  UIKit                               0x0000000107844d51 -[UIViewController loadViewIfRequired] + 1235
    15  UIKit                               0x000000010784519e -[UIViewController view] + 27
    16  UIKit                               0x000000010787713b -[UINavigationController _startCustomTransition:] + 954
    17  UIKit                               0x000000010788d894 -[UINavigationController _startDeferredTransitionIfNeeded:] + 686
    18  UIKit                               0x000000010788eb90 -[UINavigationController __viewWillLayoutSubviews] + 115
    19  UIKit                               0x0000000107ae52ae -[UILayoutContainerView layoutSubviews] + 231
    20  UIKit                               0x0000000107775551 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1331
    21  QuartzCore                          0x000000010f85f4ba -[CALayer layoutSublayers] + 153
    22  QuartzCore                          0x000000010f8635a9 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 401
    23  QuartzCore                          0x000000010f7ec1cd _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 365
    24  QuartzCore                          0x000000010f817ae4 _ZN2CA11Transaction6commitEv + 500
    25  QuartzCore                          0x000000010f818830 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 76
    26  CoreFoundation                      0x0000000106c47db7 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
    27  CoreFoundation                      0x0000000106c47d0e __CFRunLoopDoObservers + 430
    28  CoreFoundation                      0x0000000106c2c324 __CFRunLoopRun + 1572
    29  CoreFoundation                      0x0000000106c2ba89 CFRunLoopRunSpecific + 409
    30  GraphicsServices                    0x000000010e37e9c6 GSEventRunModal + 62
    31  UIKit                               0x00000001076a6d30 UIApplicationMain + 159
    32  Invite                              0x000000010419b2e7 main + 55
    33  libdyld.dylib                       0x000000010743bd81 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

group is a LinkingObjects object, then why its crashing

[bdash]

@jkmathew, if you have a question please either ask it on Stack Overflow (we monitor the realm tag) or in a new issue. Please don't just dump it on the end of an issue that's been closed for 18 months.