Re: IS-A relationships in core data
Re: IS-A relationships in core data
- Subject: Re: IS-A relationships in core data
- From: Daniel Child <email@hidden>
- Date: Tue, 20 Nov 2007 12:50:30 -0500
So in the Core Data model, are they treated as two separate tables?
My SQL is rusty, but I believe you would be defining a field in each
table linking the two. That was why I had the impulse to do this. The
classic example from my DB book had:
CLIENT (clientID, name, amtDue)
PARTNERSHIP_CLIENT (clientID, taxID, managingPartnerName, etc...)
CORPORATE_CLIENT (clientID, contactPerson, phone, etc.)
The point is that you add the key of the supertype (clientID) into
the subtype table, or vice versa.
The fact that Core Data does NOT do this makes it hard to know
exactly how things are handled. Apparently, the relations are handled
using surrogate keys (as stated in the response by Melissa Turner at
Apple), but there is no option of whether or not to set them. It is
not clear to me how to define my key fields. Supposing, for instance,
I want two fields together to be the key field, is there any way to
do that?
A related question is whether Core Data handles object-relational
scenarios. The examples I've seen show you how to set basic attribute
types (string, integer, etc.), and how to set 1-N or N-M relations.
But what if you want one of those attributes to be an object of some
kind. Do you choose NSData as the attribute type?
So one source of my confusion stems from the fact that Core Data can
store data in SQLite format, but does not really work like SQL at all.
Another question is that, while Core Data can store data in XML
format, can it really handle complex hierarchical structures like
those in dictionaries, where there may be any number of parts of
speech indicators, meanings, associated explanations or sample
sentences, etc?
If Core Data can do this, then I should go beyond the initial
tutorials and try to master it. If not, I should devote my time to
something else that will. I would love to be able to use it, however,
as doing things from scratch would be very tedious.
Thanks.
On Nov 16, 2007, at 2:29 PM, mmalc crawford wrote:
On Nov 16, 2007, at 11:06 AM, Daniel Child wrote:
When I name the two fields the same thing
Why are you doing this?
If OldEnglishWords inherits from Words, then it gets the keys from
Words...
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