Querying Core Data for Specific Attributes when Creating New Objects and returning the object if it exists, or creating a new one if it does not

Question

I have a problem checking whether a particular attribute of an Entity exists in the Core Data Database (through predicates) before creating a new object; if the object exist

Answer 1

"Is this correct?":
No. You are creating a new Person and Occasion objects whether you are using an existing person/occasion or not.
First check for existence and only if the object not already exist, insert a new one.
Alternatively, if the person/occasion exist, delete the inserted object.

"How do I retrieve the managedObjectID for person/event?":

Person* person = /*Get an existing person object*/
NSManagedObjectID* personId = person.objectID /*This is the person object ID, will work for any NSManagedObject subclass*/

To find a person that start with a string str use this predicate in a fetch request:

/*UNTESTED*/
[NSPredicate predicateWithFormat:@"(name BEGINSWITH[cd] %@)", str];

Edit:

To be more precise, you practice find or create using something like this:
(this is very limited, and only good for a single object performance-wise) (NOT TESTED)

- (NSManagedObject*) findOrCreateObjectByValue:(id)value
                                  propertyName:(NSString*)propertyName
                                    entityName:(NSString*)entityName
                                additionalInfo:(NSDictionary*)additionalInfo
                                       context:(NSManagedObjectContext*)context
                                         error:(NSError* __autoreleasing*)error
{
    NSManagedObject* res = nil;

    NSFetchRequest* r = [NSFetchRequest fetchRequestWithEntityName:entityName];
    [r setPredicate:[NSPredicate predicateWithFormat:@"%K == %@",propertyName,value]];

    NSArray* matched = [context executeFetchRequest:r
                                              error:error];

    if (matched) {
        if ([matched count] < 2) {
            res = [matched lastObject];
            if (!res) { //No existing objects found, create one
                res = [NSEntityDescription insertNewObjectForEntityForName:entityName
                                                    inManagedObjectContext:context];
                [res setValue:value
                       forKey:propertyName];
                [res setValuesForKeysWithDictionary:additionalInfo];
            }
        } else {
            if (error) {
                *error = [NSError errorWithDomain:@"some_domain"
                                             code:9999
                                         userInfo:@{@"description" : @"duplicates found"}];
            }
        }
    }

    return res;
}

So now, your save: method should look something like:
(I assume here that the person name and occasion title are held by a UITextField on the view controller [txtPersonName and txtOccasionTitle respectively] )

- (void) save:(id)sender
{
    //create a clean context so that changes could be discarded automatically on failure
    NSManagedObjectContext* context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    [context setParentContext:[self managedObjectContext]];

    //A Transaction is always created in save event, so add it to the context
    Transaction* transaction = [NSEntityDescription insertNewObjectForEntityForName:@"Transaction" inManagedObjectContext:context];

    __block NSError* error = nil;

    Person* p = (Person*)[self findOrCreateObjectByValue:self.txtPersonName.text
                                            propertyName:@"name"
                                              entityName:@"Person"
                                          additionalInfo:nil
                                                 context:context
                                                   error:&error];
    if (!p) {
        NSLog(@"Error: %@, person name: %@",error,self.txtPersonName.text);
        return;
    }

    Occasion* o = (Occasion*)[self findOrCreateObjectByValue:self.txtOccasionTitle.text
                                                propertyName:@"title"
                                                  entityName:@"Occasion"
                                              additionalInfo:nil
                                                     context:context
                                                       error:&error];
    if (!o) {
        NSLog(@"Error: %@, occasion title: %@",error,self.txtOccasionTitle.text);
        return;
    }

    transaction.whoBy = p;
    transaction.occasion = o;
    //Not sure what you are using this property for
    transaction.item = [NSEntityDescription insertNewObjectForEntityForName:@"Item"
                                                     inManagedObjectContext:context];

    NSManagedObjectContext* ctx = context;
    if ([context obtainPermanentIDsForObjects:[context.insertedObjects allObjects]
                                        error:&error])
    {
        //save your changes to the store
        __block BOOL saveSuccess = YES;
        while (ctx && saveSuccess) {
            [ctx performBlockAndWait:^{
                saveSuccess = [ctx save:&error];
            }];
            ctx = [ctx parentContext];
        }

        if (!saveSuccess) {
            NSLog(@"Could not save transaction, error: %@",error);
        }
    } else {
        NSLog(@"Could not obtain IDs for inserted objects, error: %@",error);
    }

    //Do what you have to do next
}

This is just for making things a bit clearer on what you should do to avoid duplications, and reuse existing objects.