![[info]](http://l-stat.livejournal.com/img/userinfo.gif){kind=small}
It turns out that this is somewhat straightforward to do using Core Data. Let's say you have a simple model with Song and Artist entities. I'll write it out here in a pseudo-modeling language for ease of reading:
MusicModel = {
Song = {
attribute title : string;
attribute duration : float;
to-one-relationship artist : Artist,
inverse : songs,
delete-rule : nullify;
userInfo = { };
};
Artist = {
attribute name : string;
to-many-relationship songs : Song,
inverse : artist,
delete-rule : cascade;
userInfo = { };
};
};xcdatamodel files in Xcode, one with each entity, and wire the relationships together after loading them and merging them with +[NSManagedObjectModel modelByMergingModels:]. Except that won't work: Relationships with no destination entity won't be compiled by the model compiler.What else might you try? You could try just putting dummy entities in for relationships to point to. However, merging models will fail then, because NSManagedObjetModel won't merge models that have entity name collisions.
It turns out, though, that you can merge models very easily by hand, by taking advantage of the way Core Data's model-description objects handle the
NSCopying protocol. All you have to do is create your destination model, loop through every entity in each of your source models, and copy every entity that you haven't tagged as a stand-in using a special key in their userInfo dictionary.Why does this work? The trick is that before you tell a persistent store coordinator to use a model, that model is mutable and references relationship destination entities and inverse relationships by name. So you can have only a minimal representation of Artist in one model, and a minimal representation of Song in another model:
SongModel = {
Song = {
attribute title : string;
attribute duration : float;
to-one-relationship artist : Artist,
inverse : songs,
delete-rule : nullify;
userInfo = { };
};
Artist = {
/* Note no attributes. */
to-many-relationship songs : Song,
inverse : artist,
delete-rule : cascade;
userInfo = { IsPlaceholder = YES; };
};
};
ArtistModel = {
Song = {
/* Note no attributes. */
to-one-relationship artist : Artist,
inverse : songs,
delete-rule : nullify;
userInfo = { IsPlaceholder = YES; };
};
Artist = {
attribute name : string;
to-many-relationship songs : Song,
inverse : artist,
delete-rule : cascade;
userInfo = { };
};
};- (NSManagedObjectModel *)mergeModelsReplacingDuplicates:(NSArray *)models {
NSManagedObjectModel *mergedModel = [[[NSManagedObjectModel alloc] init] autorelease];
// General strategy: For each model, copy its non-placeholder entities
// and add them to the merged model. Placeholder entities are identified
// by a MyRealEntity key in their userInfo (which names their real entity,
// though their mere existence is sufficient for the merging).
NSMutableArray *mergedModelEntities = [NSMutableArray arrayWithCapacity:0];
for (NSManagedObjectModel *model in models) {
for (NSEntityDescription *entity in [model entities]) {
if ([[[entity userInfo] objectForKey:@"IsPlaceholder"] boolValue]) {
// Ignore placeholder.
} else {
NSEntityDescription *newEntity = [entity copy];
[mergedModelEntities addObject:newEntity];
[newEntity release];
}
}
}
[mergedModel setEntities:mergedModelEntities];
return mergedModel;
}