Re: Calculated Field examples for Core Data
Re: Calculated Field examples for Core Data
- Subject: Re: Calculated Field examples for Core Data
- From: Clayton Leitch <email@hidden>
- Date: Sun, 14 Aug 2005 10:41:37 -0400
Well, I found a document that talked about derived in core data,
NSPersistentDocumentTutorial.pdf. However, it talks about taking
string variables and concatenating them together to give a person's
full name. However, it is essentially the same problem that I have.
Following the instructions, to the letter, I made a class file from
the data modeling interface, an NSManaged object is the superclass.
When I compiled the file I received the errors that are described below:
If I uncomment the self setKeys lines I get a selector unrecognized
error when I try to run the project. I have tried various means to
do the subtraction including the decimalNumberBySubtracting method.
All either give me an error or a warning. I also tried
primitiveValueForKey with no success.
Any Ideas?
On Aug 14, 2005, at 4:00 AM, mmalcolm crawford wrote:
On Aug 13, 2005, at 4:49 AM, Clayton Leitch wrote:
I am using cocoa bindings to display everything now. As I said
everything works perfectly and I use Core Data to manage the data
store. The first thing I tried was to look at if there was some
way to access the data values in the various fields. This led me
to the custom accessor page on Apple's site. However, when I
attempted to get value for key on the various elements of the
managed objects in the core data structure, I was never able to
have the value appear.
How are you trying to retrieve a value? This document <http://
developer.apple.com/documentation/Cocoa/Conceptual/CoreData/
Articles/cdAccessorMethods.html#//apple_ref/doc/uid/TP40002154>
shows how to write accessor methods, but also states,
"There is no need to write custom accessor methods for properties
that are defined in the entity of a managed object’s corresponding
managed object model. You can access properties using standard key-
value coding methods such as valueForKey:."
Next I considered that I needed a controller object to retrieve
the values.
If you're using bindings, then you should use a controller to
mediate between your user interface and the managed objects.
I was unsuccessful there too. Then I thought if there was an
example available that showed this type of operation, that might
give me an idea. To google I went. I found several code
fragments but nothing that represented a complete project. How do
I access the elements of the managed objects with value for key?
id attributeValue = [anObject valueForKey:@"attributeName"];
This is given in numerous places in the documentation.
Again, none of this though is specific to Core Data, it's general
to Cocoa and Cocoa Bindings -- for examples see <http://
developer.apple.com/documentation/Cocoa/Conceptual/CocoaBindings/
index.html>. For more about key-value coding, see <http://
developer.apple.com/documentation/Cocoa/Conceptual/KeyValueCoding/
index.html>, and for key-value observing see <http://
developer.apple.com/documentation/Cocoa/Conceptual/
KeyValueObserving/index.html>, in particular "Registering Dependent
Keys" -- for a Core Data-based example, this may be useful: <http://
developer.apple.com/documentation/Cocoa/Conceptual/
NSPersistentDocumentTutorial/03_CustomClass/chapter_4_section_3.html>.
mmalc
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