Database Reference
In-Depth Information
Once the operation is started, we must perform a couple of start-up tasks.
First, since the operation is going to be executed on a separate thread, we
need to construct a new
NSManagedObjectContext
.
- (
void
)main
{
ZAssert([self completionBlock], @
"No completion block set"
);
NSManagedObjectContext *localMOC = nil;
localMOC = [[NSManagedObjectContext alloc] init];
NSPersistentStoreCoordinator *psc = nil;
psc = [self persistentStoreCoordinator];
[localMOC setPersistentStoreCoordinator:psc];
Now that we have a local
NSManagedObjectContext
, we need to obtain a local copy
of the recipe. A call to
-objectWithID:
returns a local copy of the
NSManagedObject
.
We also confirm that it did return an object for us.
-objectWithID:
is a method
on
NSManagedObjectContext
that will return a context-local copy of the associated
NSManagedObject
. If the
NSManagedObject
is already loaded into the context, then
it will return the object; otherwise, it will attempt to fetch it from the
NSPersis-
tentStoreCoordinator
.
Once we have a local copy of the recipe, we need to turn it into a JSON
structure. To do that, we must first convert the
NSManagedObject
into a dictionary.
However, we don't want to just grab the top-level object; we also want the
author, the ingredients, and so on. To get all of those, we'll have to do a bit
of recursive work.
- (NSDictionary*)moToDictionary:(NSManagedObject*)mo
{
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
if
(!mo)
return
dict;
NSEntityDescription *entity = [mo entity];
NSArray *attributeKeys = [[entity attributesByName] allKeys];
NSDictionary *values = [mo dictionaryWithValuesForKeys:attributeKeys];
[dict addEntriesFromDictionary:values];
The first part of the conversion involves grabbing all of the attributes (strings,
numbers, dates) from the
NSManagedObject
and placing them into an
NSMutable-
Dictionary
. By utilizing
KVC
, we can do this with a small amount of code. We
first ask the
NSEntityDescription
for all of the attribute names, and then we call
-dictionaryWithValuesForKeys:
on the
NSManagedObject
. This returns a dictionary with
all of the keys and values for the attributes. We then add the resulting dictio-
nary into our
NSMutableDictionary
.
