New feature available on the iOS/Mac master branch, as described in issue #606
<https://github.com/couchbase/couchbase-lite-ios/issues/606>:
Added CBLModel support for computed inverse-relation properties (a typical ORM
feature, also found in Core Data.)
For example, if the Album class has a relation @property Artist* artist (i.e.
the JSON property value is the docID of a document corresponding to an Artist
model), then it should be possible to define a property on Artist like
@property (readonly) NSArray* albums, whose value is an array of the
Albumobjects whose artist property points to the receiver.
Behind the scenes, there's no JSON property backing this. Its value is computed
on demand by querying a view that indexes the artist property of every album
document. (If this turns out to be too slow we can cache the value until the
view index changes.)
Caveats:
The property value is recomputed on every call.
These properties are not KV-observable (that would be great to add later, but
it would require something like a LiveQuery, and would have to be carefully
optimized to be scaleable.)
To define a property like this, you have to do three things:
1. Define the [dynamic] property itself, with type NSArray*.
2. Implement a class method +propertyItemClass that returns the class that has
the inverse relation, i.e. the class of the items in the array to be returned.
3. Implement a class method +propertyInverseRelation that returns a string
naming the relation property in the other class that should be queried.
In the example:
@interface Artist
@property (readonly) NSArray* albums;
...
@implementation Artist
...
@dynamic albums;
+ (Class) albumsItemClass { return [Album class]; }
+ (NSString*) albumsInverseRelation { return @"artist"; }
...
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