Default value for a Relationship in Core Data - objective-c

Question

I created a model with two entities. Entity A has a relationship (to-one) to entity B. My problem is, that I have no idea how to set up a default value for the relationship. If I add a new entry to the entity A I get "no value" for the value of the relationship (action sent to the corresponding NSArrayControler or [newObjectEntityA initWithEntity:....] . What I'm missing is a way to assign a default value for relationships like for attributes.
Example:
Entity A
attrib a (has a default value)
attrib b (has a default value)
relation to B (has no default value)
Of course a workaround is something like this:
[newObjectEntityA setValue: defaultValueInEntityB forKey:#"relationToB"];
but I think there must be a more convenient solution.
Thanks in advance,
Thomas

Answer 1

You should override awakeFromInsert instead, this method is designed for adding in default values and other startup operations.

Answer 2

I would create a Factory, that creates new objects of A and preconfigure it according your needs.

Answer 3

okay... I got the problem solved (more or less). What I did is to subclass the NSManagedObject and I overriden the method -(void) initWithEntity:insertIntoManagedObjectContext:
#implementation Locos
static NSManagedObject *defaultType;
- (id)initWithEntity:(NSEntityDescription *)entity insertIntoManagedObjectContext:(NSManagedObjectContext *)context
{
self=[super initWithEntity:entity insertIntoManagedObjectContext:context];
if(self==nil)
{
[self release];
return(nil);
}
if(defaultType==nil)
{
NSEntityDescription* locoType=[NSEntityDescription entityForName:#"LocoType" inManagedObjectContext:context];
if(locoType==nil) return(self);
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:locoType];
NSPredicate *predicate=[NSPredicate predicateWithFormat:#"Default==TRUE"];
[request setPredicate:predicate];
[context executeFetchRequest:request error:nil];
NSArray *array=[context executeFetchRequest:request error:nil];
defaultType=[array objectAtIndex:0];
}
if(defaultType!=nil)
{
[self setPrimitiveValue: defaultType forKey:#"Type"];
}
return(self);
}
#end
It sets the relationship "Type" in the entity "Locos" to the first record in the entity "LocoType" which has the attribute "Default" set to true. It works, but there is something I don't understand. I get a very strange effect if I simplify the method to:
#implementation Locos
- (id)initWithEntity:(NSEntityDescription *)entity insertIntoManagedObjectContext:(NSManagedObjectContext *)context
{
self=[super initWithEntity:entity insertIntoManagedObjectContext:context];
if(self==nil)
{
[self release];
return(nil);
}
NSEntityDescription* locoType=[NSEntityDescription entityForName:#"LocoType" inManagedObjectContext:context];
if(locoType==nil) return(self);
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:locoType];
NSPredicate *predicate=[NSPredicate predicateWithFormat:#"Default==TRUE"];
[request setPredicate:predicate];
[context executeFetchRequest:request error:nil];
NSArray *array=[context executeFetchRequest:request error:nil];
[self setPrimitiveValue: [array objectAtIndex:0] forKey:#"Type"];
return(self);
}
#end
in this case, if I push a button which is bound to add: of an array controller (which is bound to the entity 'Locos') I always get TWO new entries! I run the debugger and the method initWithEntry is called only ONCE. I also get rid of the problem as soon as I remove the line
[context executeFetchRequest:request error:nil];
It also doesn't happen if the fetch is called only once as in the first code example where I store the record i the static variable.
Does anybody has any idea what happens there??