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Re: How do you store an NSArray as an attribute in a Core Data entity
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Re: How do you store an NSArray as an attribute in a Core Data entity


  • Subject: Re: How do you store an NSArray as an attribute in a Core Data entity
  • From: Chris Hanson <email@hidden>
  • Date: Thu, 19 Mar 2009 02:28:37 -0700

On Mar 19, 2009, at 1:26 AM, email@hidden wrote:

I want to store a list of Strings as an attribute of an entity in Core Data but there doesn't appear to be a way to use NSArray (or NSSet or NSDictionary either) as an attribute.?Please can someone explain how you use an NSArray or NSDictionary as an attribute of a Core Data entity?

In general, if you want to have multiple things as an attribute of a Core Data entity, what you really want is a to-many relationship with another entity.


Let's take tags as an example. Say I'm creating a weblog editor and I want the ability to tag my posts. I could model this as a string attribute with some specific format (space- or comma-separated), or as a transformed attribute. However, that doesn't get me the ability to do things like say "show all the posts with this tag" or even "auto- complete known tags." It also leads to persistent store bloat because I'm breaking a cardinal rule of relational data, "store each piece of data once," because each tagged post winds up with a copy of the tag's text.

Instead of having an attribute, I would model a to-many relationship from my Post entity to my new Tag entity, with a to-many inverse relationship from my Tag entity to my BlogPost entity. This way:

Each Tag instance only exists once in the persistent store.
I can traverse the object graph between Posts and Tags, and between Tags and Posts, with ease.
I can perform interesting queries across both Posts and Tags to improve my user experience (e.g. support auto-completion or generate tag clouds).
Not having Tags stored directly on Posts means if I'm just dealing with a Post, Core Data won't bother fetching the Tags too.


Entities and relationships are good, hope this helps you leverage them!

  -- Chris

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