CODEWITHSUNDEEP
ioscore-data
pazustep
pazustep

Reputation: 441

Core Data and multiple to-many relationships

I have the folowing Core Data model:

Core Data Model

That is, ach ItemCategory belongs to a project (reverse: categories), and can optionally belong to a parent ItemCategory (category, reverse subcategories) — please ignore the rest.

Thing is, apparently I can't set both the project and the parent category for an ItemCategory. Every time I set one relationship the other relationship is set to nil. I've tried every combination of delete rule, but nothing seems to work.

Is there a fundamental Core Data restriction I'm not aware, or am I doing something wrong?

Upvotes: 0

Views: 251

Answers (2)

Daniel Eggert
Daniel Eggert

Reputation: 6715

You can have multiple to-many relationships between two entities. The important thing is that each relationship has its own inverse. Let's say we have entity A and B and we want two to-many relationships between them. You can do that.

We'll start out with

A <<----->> B

Your A entity will have a relationship which we'll call bs with an inverse which we'll call as. So far everything is normal.

Nothing prevents you from adding another relationship, though:

  <<----->>
A           B
  <<----->>

For the 2nd relationship, we'll name the relationship from A to B as otherBs and its inverse for otherAs.

Now A has two relationships to B, namely bs and otherBs. And the same goes for the relationships the other way.

Upvotes: 1

Phillip Mills
Phillip Mills

Reputation: 31016

I'll vote for "doing something wrong". :) I just tried the simplest form of this with a Project, an ItemCategory, no attributes, no other entities, their own relationships...and it doesn't show that symptom. This...

   Project *p = [NSEntityDescription insertNewObjectForEntityForName:@"Project" inManagedObjectContext:self.managedObjectContext];
   ItemCategory *c1 = [NSEntityDescription insertNewObjectForEntityForName:@"ItemCategory" inManagedObjectContext:self.managedObjectContext];
   ItemCategory *c2 = [NSEntityDescription insertNewObjectForEntityForName:@"ItemCategory" inManagedObjectContext:self.managedObjectContext];
   c1.category = c2;
   c1.project = p;

...gives...

(lldb) po c1.category (ItemCategory *) $2 = 0x06b98bd0 <ItemCategory: 0x6b98bd0> (entity: ItemCategory; id: 0x6b98ee0 <x-coredata:///ItemCategory/tB2F1219F-4FA5-48DA-871E-D9F9DC8E33E34> ; data: {
    category = nil;
    project = nil;
    subcategories =     (
        "0x6b82280 <x-coredata:///ItemCategory/tB2F1219F-4FA5-48DA-871E-D9F9DC8E33E33>"
    ); })
(lldb) po c1.project (Project *) $3 = 0x06b97a20 <Project: 0x6b97a20> (entity: Project; id: 0x6b97a70 <x-coredata:///Project/tB2F1219F-4FA5-48DA-871E-D9F9DC8E33E32> ; data: {
    categories =     (
        "0x6b82280 <x-coredata:///ItemCategory/tB2F1219F-4FA5-48DA-871E-D9F9DC8E33E33>"
    ); })

Unfortunately, this doesn't shed any light on what may be wrong but it doesn't seem inherently a restriction.

Upvotes: 0

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