How does one fetch a set of unique managed objects by requesting a specific attribute to be checked.
e.g. A number of people objects and I would like to retrieve all the unique names, one managed object for each unique name, sorted by name.
What about using a request like this
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Person" inManagedObjectContext:managedObjectContext];
request.entity = entity;
request.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:#"name"]];
request.returnsDistinctResults = YES;
request.resultType = NSDictionaryResultType;
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptors]];
NSError *error = nil;
NSArray *distinctResults = [managedObjectContext executeFetchRequest:request error:&error];
// Use distinctResults
Try and let me know.
P.S. Code is ARC enabled. If you are not using it, call release when necessary.
Related
How do I setup a fetch request to only pull the data from an entity's attribute with one particular value? This is the basic code I've used before.
-(void)fetchResults
{
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:self.entityDescription.name];
NSString *cacheName = [self.entityDescription.name stringByAppendingString:#"Cache"];
// predicate code
if (self.predicate != nil) {
[fetchRequest setPredicate:self.predicate];
}
// end of predicate code
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:cacheName];
BOOL success;
NSError *error;
success = [self.fetchedResultsController performFetch:&error];
if (!success)
{
NSLog(#"%#", [error localizedDescription]);
}
}
I've been looking at this page: http://bit.ly/KevYwR is this the right direction?
Do I need to use NSPredicate or can I do without?
Thanks for any help, point in the right direction, etc.
Setting up a NSFetchRequest is equivalent to a SELECT statetement in SQL language.
Here a simple example:
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:#"EntityName" inManagedObjectContext:moc]];
NSError *error = nil;
NSArray *results = [moc executeFetchRequest:request error:&error];
// error handling code
The array results contains all the managed objects contained within the sqlite file. If you want to grab a specific object (or more objects) you need to use a predicate with that request. For example:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"attribute == %#", #"Some Value"];
[request setPredicate:predicate];
In this case results contains the objects where attribute is equal to Some Value. Setting a predicate is equal to put the WHERE clause in a SQL statement.
Note
I suppose that the name of the entity is EntityName and its property is called attribute which is of type string.
For further info I suggest you to read the Core Data programming guide and NSFecthRequest class reference.
http://developer.apple.com/library/iOS/#documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html
https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html
Hope it helps.
I have two entities, Job and Client. The relationship from job to client is called clientOfJob which is a many-to-one relationship:
Job<<-->Client
I am trying to understand how I would sort my fetch results into sections for the table view, with the sections based on client first names.
This is my first attempt to use sectionNameForKeyPath, but I am using sample code from CoreDataBooks, and so far, substituting some of that code has eliminated some serious problems I had earlier today.
Here is the code I tried. I didn't think it would work, but I'm not sure what I need to use for the client sort descriptor key and for the sectionNameKeyPath.
By the way, the code works well with only the jobSortDescriptor (without the clientSortDescriptor) and with sectionNameKeyPath set to nil. Then it just sorts by job description. I can get at the related client entity information, putting client name information into the detailedTextLabel of each cell while I have the job description in the textLabel of the cell.
- (NSFetchedResultsController *) fetchedResultsController {
if (fetchedResultsController != nil) {
return fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription * entity = [NSEntityDescription entityForName:#"Job" inManagedObjectContext:dataInterface.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:10];
NSSortDescriptor *clientSortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"clientOfJob.firstName" ascending:YES];
NSSortDescriptor *jobSortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"jobDescription" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:clientSortDescriptor, jobSortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:dataInterface.managedObjectContext sectionNameKeyPath:#"clientOfJob.firstName" cacheName:#"Jobs"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
[aFetchedResultsController release];
[fetchRequest release];
[jobSortDescriptor release];
[clientSortDescriptor release];
[sortDescriptors release];
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
NSLog(#"Unresolved Error %#, %#", error, [error userInfo]);
abort();
}
return fetchedResultsController;
}
I am trying to use NSSortDescriptor in Core Data to fetch my records. Array of modal-objects doesn't get affected by sort descriptor. It gives records in same order.Here is my code:
NSManagedObjectContext *moc=[self managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"To_Do" inManagedObjectContext:moc];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"repeatDate" ascending:NO];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];
[sortDescriptor release];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:entity];
NSError * error = nil;
NSArray *arrEntity = nil;
arrEntity=[moc executeFetchRequest:fetchRequest error:&error];
The code looks fine and should work. Since it does not there a couple of possible causes.
The unsorted fetch order might be the same as sorted order. This can happen sometimes if you create objects in series and use a a key like a creation date or the like.
Your keys might all have the same value. This can happen if you have a default value.
You have the wrong key or misspelled its name. You should get a complaint but it won't crash if it can't find the key.
I had same problem if I had done like this
fetchRequest.sortDescriptors.append("key")
But if I use
fetchRequest.sortDescriptors = ["key"]
Then it works fine
I have an entity ("Settings") and I want to get all the values in just one field ("Status").I am using coredata. Can anyone help me, please?
Declare this in .h file:
NSMutableArray *eventArray;
and in .m file
- (void)fetchRecords {
// Setup the fetch request
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Define our table/entity to use
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Settings" inManagedObjectContext:managedObjectContext];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"Status" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:entity];
[fetchRequest setEntity:entity];
NSError *error;
// Get array of results.
NSMutableArray *theResults = [[managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];
if (!theResults) {
// Handle the error.
// This is a serious error and should advise the user to restart the application
}
// Grab unique neighborhoods through NSSet.
NSSet *uniqueElements = [NSSet setWithArray:[theResults valueForKey:#"Status"]];
// Dump NSSet uniques into new array.
NSMutableArray *sortedResults = [[NSMutableArray alloc] initWithArray:[uniqueElements allObjects]];
for(int i = 0; i < [sortedResults count];i++)
{
NSLog(#"%d. %#", i+1, [sortedResults objectAtIndex:i]);
}
// Save our fetched data to an array
[self setEventArray: sortedResults];
}
At first, u can get Array of values:
NSFetchRequest *requestSettings = [[NSFetchRequest alloc] init];
[requestCodesList setEntity:[NSEntityDescription entityForName:#"Setting" inManagedObjectContext:managedObjectContext]];
NSArray *setting = [managedObjectContext executeFetchRequest:requestCodesList error:&error];
if (error) NSLog(#"Failed to executeFetchRequest to data store: %#", [error localizedDescription]);
Next, u transfer u array in NSString:
ComponentsJoinedByString: Constructs
and returns an NSString object that is
the result of interposing a given
separator between the elements of the
array.
(NSString *)componentsJoinedByString:(NSString *)separator
code is:
NSString *settingChanged = [setting componentsJoinedByString:#","];
And after u can using this anywhere in u UI (binding, setStringValue e.t.c)
Welcome
i use Core Data to store datas. i need such a method which returns only the last 7 elements of entity. my question is how should i modify this code ( it fetchs all of elements, but i need only last 7)
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Trip" inManagedObjectContext:managedObjectContext];
NSFetchRequest *request = [[ NSFetchRequest alloc] init];
[request setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"distance" ascending:NO];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
[request setSortDescriptors:sortDescriptors];
[sortDescriptor release];
NSError *error;
tripArray = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
Define last? Core Data does not have a concept of order internally. If you mean by the farthest away based on your distance property then you can do the following:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Trip" inManagedObjectContext:managedObjectContext];
NSFetchRequest *request = [[ NSFetchRequest alloc] init];
[request setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"distance" ascending:NO];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
[request setSortDescriptors:sortDescriptors];
[request setFetchLimit:7];
[sortDescriptor release];
NSError *error;
NSArray *tripArray = [managedObjectContext executeFetchRequest:request error:&error];
Note that the addition of the -setFetchLimit: will cause this request to only return 7 results. It will return the "first" 7 based on your sort. So if you want the closest, reverse the ascending: portion of your sort.
-mutableCopy
There is absolutely no point in calling -mutableCopy on the NSArray that is returned from -executeFetchRequest: error:. Adding objects to that NSArray will not add them to Core Data and removing them from that NSArray will not remove them from Core Data. Therefore it has absolutely no value and is just wasteful.
Do you remember where you saw that? I have been trying to track it down for a while now.