On Mar 19, 2009, at 1:26 AM, tmow...@talktalk.net 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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