core data fetch data order by id - objective-c

Question

I'm trying to get allways the same order for my fetch data, because all the time I request, he get another sorted array so I'm trying to order by ID but I couldn't.
My situation:
I have 2 models, FORM and FORMCOMPONENT
1 FORM have many FORMCOMPONENT
when I do this:
Form *form = ...;
[form formComponents]; //I get all the components but each time I run I get with a differente order
How I suppose order this if I don't have any field for ORDER? On android I did this with the ID.. that's why I don't have any field with this proposal.
I tryed to order in a array with sortWithComparator but I can't do this because I'm not allowed to get the number ID (only the number, not the entire string he give me with [obj objectID] ). This is funny because when I use sqlite3 for see the database only have the number there.
Another way I thought about is get the FORMCOMPONENTs direct with a new request, not by FORM one ([form formComponents]), like this:
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"FormComponent" inManagedObjectContext:[appDelegate managedObjectContext]];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
//SOME PREDICATE WITH THE FORM
[request setPredicate:predicate];
NSError *error = nil;
NSMutableArray *arrayComps = (NSMutableArray*) [[appDelegate managedObjectContext] executeFetchRequest:request error:&error];
so, anyone have an idea? Thanks.
--- EDIT ---
PS: I don't want to avoid the problem creating another field for order. If I don't have any other solution so I will do this.

Answer 1

You can sort by objectID by using a sort descriptor with self as the sort key. The underlying SQL will have ORDER BY t0.Z_PK, Z_PK being the internal ID of the record (as found as the last component of the object's NSManagedObjectID).

Answer 2

Core data is no database.
Internally Core Data uses PK, of course. But Core Data is no database, it is a system to model a graph. Consequently no "ID information" is published. Looking for an ID is anti-conceptional.
If your data does not have a "natural order", there should be nothing wrong in getting a random order. If it has a natural order, simply use sort descriptors.
If you need an order "creation date" simply add a property creationDate and set it in -awakeFromInsert. (BTW: AFAIK it is an urban legend, that PK always raises. A PK can be something else than autoincrement.)

Answer 3

Core Data doesn't expose any kind of SQL-style numeric ID that you could use for sorting. If you don't want to add an attribute to sort on, the easiest thing might be to configure this relationship as being ordered. Then you get an NSOrderedSet for the relationship, and you can keep individual instances in whatever order you want. Make the relationship ordered in the Core Data model editor, and use mutableOrderedSetValueForKey: when you want to add a new instance to the relationship.

Answer 4

The recommended way it to add a field for the ID/sort order you want to give to your components.
Alternatively you could make the relationship ordered by checking the checkbox for that in the CoreData editor. This has some more overhead, but is the only option if components need to appear in multiple lists.
Meddling with SQLite directly isn't a smart approach.

Answer 5

Thanks for the help. I found what I'm looking for (with this I can order for example)
I need to get PK direct from sql, if anyone needs do this, here is the code:
Just don't forget what our friend Tom said: "That's not how Core Data works"
-(NSMutableArray*) readDatabase {
NSLog(#"READ");
sqlite3 *database;
NSMutableArray *components = [[NSMutableArray alloc] init];
NSArray *documentPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDir = [documentPaths objectAtIndex:0];
NSString *databasePath = [documentsDir stringByAppendingPathComponent:#"Model.sqlite"];
// Open the database from the users filessytem
if(sqlite3_open([databasePath UTF8String], &database) == SQLITE_OK) {
NSLog(#"SQLITE_OK");
// Setup the SQL Statement and compile it for faster access
const char *sqlStatement = "select * from zformcomponent";
sqlite3_stmt *compiledStatement;
if(sqlite3_prepare_v2(database, sqlStatement, -1, &compiledStatement, NULL) == SQLITE_OK) {
// Loop through the results and add them to the feeds array
while(sqlite3_step(compiledStatement) == SQLITE_ROW) {
// Read the data from the result row
int aPK = sqlite3_column_int(compiledStatement, 0);
//NSString *aDescription = [NSString stringWithUTF8String: (char *)sqlite3_column_text(compiledStatement, 2)];
//NSString *aImageUrl = [NSString stringWithUTF8String:(char *)sqlite3_column_text(compiledStatement, 3)];
NSLog(#"PK: %d",aPK);
//Animal *animal = [[Animal alloc] initWithName:aName description:aDescription url:aImageUrl];
//[animals addObject:animal];
}
}
sqlite3_finalize(compiledStatement);
}
sqlite3_close(database);
return components;
}