Re: Core Data getting many data from relationship.
Re: Core Data getting many data from relationship.
- Subject: Re: Core Data getting many data from relationship.
- From: Chris Hanson <email@hidden>
- Date: Fri, 6 Oct 2006 13:58:48 -0700
On Oct 6, 2006, at 1:28 PM, David Alter wrote:
I think this is a very basic question and I'm just not finding the
answer. I want to get the many data for a given relationship. Lets
say I have a Department and I want to get all its Employees.
Department has a relationship called employees. Is this what I do?
id result = [aDepertment valueForKey:@"employees"];
Yes, this should be spelled out in the Core Data documentation. All
basic property (attribute, to-one or to-many relationship, or fetched
property) manipulation can be performed using key-value coding.
What do I get back? Is it an array, a set, or some other kind of
container?
You get an NSSet back from that method. Since it's an NSSet, you can
only access the to-many relationship that way, not manipulate it.
Should I be using mutableArrayValueForKey from NSKeyValueCoding
protocal?
Actually, to manipulate a relationship rather than just access it, you
should use -mutableSetValueForKey:. Core Data follows the relational
model and thus relationships are unordered; order is imposed upon
data, not inherent in it.
-mutableSetValueForKey: will return an NSMutableSet that represents
the relationship, and which you can manipulate in order to manipulate
the relationship. All necessary inverse relationships etc. will be
maintained while you perform this manipulation, and any accessor/
mutator methods defined for the relationship will also be invoked as
appropriate.
I have been reading the Core Data docs and all the examples show
situations that return only one object.
Please file a bug report or use the feedback form in the web site
documentation to list the specific examples you're looking at, and
that you would like to see examples of relationship manipulation in
them as well.
-- Chris
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