I need to fetch my database and order it by id. So for example, i have 2 columns in database, database_id and content. When I NSLOG last line i get all rows of "content" column. But I need to sort those "content" rows based on database_id which is 1,2,3,4,5,6 (ascending). Help ???
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *ent = [NSEntityDescription entityForName:#"databaseName" inManagedObjectContext:self.managedObjectContext];
[request setEntity:ent];
NSSortDescriptor *srt = [NSSortDescriptor sortDescriptorWithKey:#"database_id" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:srt]];
NSError *error;
NSArray *result = [self.managedObjectContext executeFetchRequest:request error:&error];
if (!result)
{
//error
}
NSMutableArray *myArray = [[NSMutableArray alloc] initWithArray:[result valueForKey:#"content"]];
Your database_id is stored as a string and therefore the ids are sorted as strings:
1, 11, 15, 2, 3, 5
The best solution would be to store the database_id as a number (e.g. "Integer 32") in the
database.
As a workaround, you can keep your strings, and use a special compare function in the
sort descriptor:
NSSortDescriptor *srt = [NSSortDescriptor sortDescriptorWithKey:#"database_id"
ascending:YES
selector:#selector(localizedStandardCompare:)];
localizedStandardCompare does a "Finder-like" comparison. In particular, strings containing
numbers are sorted according to their numerical value.
Related
In my CoreData application I want display the last inserted stdId in Table Student. Is there any easy way to retrieve the last inserted value in a column ?
It's quite simple if in your Student entity you add a property of type NSDate like createdAt.
When you insert a new Student, you need to pass the current date for that property.
[aStudent setValue:[NSDate date] forKey:#"createdAt"];
or
aStudent.createdAt = [NSDate date];
if you have created a subclass of Entity.
Once done, you can fetch against Student using a limit of 1 and a sort descriptor with ascending value NO.
NSFetchRequest* request = [NSFecthRequest fetchRequestWithEntityName:#"Student"];
[request setFetchLimit:1];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"createdAt" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
NSError *error;
NSArray *results = [context executeFetchRequest:request error:&error];
NSManagedObject *latestAddedStudent = [results objectAtIndex:0];
Hope that helps.
I am trying to count the number of objects in my core data , so that I can tell how many rows are needed my table view controller, but I am stuck with counting of NS ARRAY, here is the code
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:#"Data" inManagedObjectContext:context];
NSSortDescriptor *sortDescriptor =
[[NSSortDescriptor alloc] initWithKey:#"date"
ascending:YES
selector:#selector(localizedCaseInsensitiveCompare:)];
request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
NSError *error = nil;
NSArray *temp=[context executeFetchRequest:request error:&error];
I am doing it in Data class(sub class of nsmanged object), the issue is when I reach
NSLog(#"%#",[temp count]);
I get ‘Program received signal:”EXC_BAD_ACCESS”’
I dnt get what is wrong, I am putting my data into a ns array and than trying to count them . help in this regard will be greatly appreciated .
[temp count] return a number , not a string
so you need
NSLog(#"%u",[temp count]);
look at this
I've got a column with integer values in CoreData. While retrieving results from it, I want the column values to be subtracted with a number.
Something like: columnValue - someNumber (this number is entered by user)
I know I may have to use NSPredicate for this, but am unaware if there's a function or syntax for it.
The alternate right now I have is to iterate all column values and subtract with 'someNumber'. But I think there should be a better and efficient way to do this.
Edit: Code from #salo.dm 's answer
- (NSDictionary *)myFetchResults {
//Predicate works fine
NSPredicate *predicate = [NSCompoundPredicate andPredicateWithSubpredicates:[NSArray arrayWithObjects:pred1, pred2, nil]];
/*Sort Descroptor - Sorting by 4 columns*/
NSSortDescriptor *sortDesc1 = [NSSortDescriptor sortDescriptorWithKey:#"Column1" ascending:YES];
NSSortDescriptor *sortDesc2 = [NSSortDescriptor sortDescriptorWithKey:#"Column2" ascending:YES];
NSSortDescriptor *sortDesc3 = [NSSortDescriptor sortDescriptorWithKey:#"Column3" ascending:YES];
NSSortDescriptor *sortDesc4 = [NSSortDescriptor sortDescriptorWithKey:#"Column4" ascending:YES];
/*Get Data*/
MyAppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [appDelegate managedObjectContext];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"TableName" inManagedObjectContext:context];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];
[fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sortDesc1, sortDesc2, sortDesc3, sortDesc4, nil]];
NSArray *listData = [context executeFetchRequest:fetchRequest error:nil];
/*Create subtract expression*/
NSExpressionDescription *subExp1 = [[NSExpressionDescription alloc] init];
[subExpLatitude setName:#"subtraction1"];
[subExpLatitude setExpression:[NSExpression expressionForFunction:#"from:subtract:"
arguments:[NSArray arrayWithObjects:
[NSExpression expressionForKeyPath:#"Column3"],
[NSExpression expressionForConstantValue:[NSNumber numberWithDouble:someNumber1]],
nil]]];
[subExp1 setExpressionResultType:NSDoubleAttributeType];
NSExpressionDescription *subExp2 = [[NSExpressionDescription alloc] init];
[subExpLongitude setName:#"subtraction2"];
[subExpLongitude setExpression:[NSExpression expressionForFunction:#"from:subtract:"
arguments:[NSArray arrayWithObjects:
[NSExpression expressionForKeyPath:#"Column4"],
[NSExpression expressionForConstantValue:[NSNumber numberWithDouble:someNumber2]],
nil]]];
[subExp2 setExpressionResultType:NSDoubleAttributeType];
/*Get difference data*/
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:subExp1, subExp2, nil]];
NSArray *listDifference = [context executeFetchRequest:fetchRequest error:nil];
NSLog(#"Subtraction 1: %#", [[listDifference objectAtIndex:0] objectForKey:#"subtraction1"]);
NSLog(#"Subtraction 2: %#", [[listDifference objectAtIndex:0] objectForKey:#"subtraction2"]);
NSMutableDictionary *dictResult;
[dictResult setObject:listData forKey:#"Data"]
[dictResult setObject:listDifference forKey:#"Difference"]
return dictResult;
}
Edit: Get coredata object
This doesn't work.
NSExpressionDescription *expEntity = [[NSExpressionDescription alloc] init];
[expEntity setName:#"TableNameEntity"];
[expEntity setExpression:[NSExpression expressionForKeyPath:#"objectID"]]; //Searches for a column for the name specified
[expEntity setExpressionResultType:NSObjectIDAttributeType];}
Had to change it to below to get it working (Assuming this is the correct way)
NSExpressionDescription *expEntity = [[NSExpressionDescription alloc] init];
[expEntity setName:#"TableNameEntity"];
[expEntity setExpression:[NSExpression expressionForEvaluatedObject]];
[expEntity setExpressionResultType:NSObjectIDAttributeType];
I added expEntity to the setPropertiesToFetch list. Now I get two values in the dictionary.
{
TableNameEntity = "0x5e22120 <x-coredata://1A659A52-9321-4ACD-992B-04F20E7BDCED/TableNameEntity/p1640>";
subtractionValue = "-24.13";
}
When I try to retrieve and access TableNameEntity from the dictionary, the app crashes.
TableNameEntity *tableEntity = (TableNameEntity *)[dict objectForKey:#"TableNameEntity"];
tableEntity.column1 //This is not the exact code. But this operation crashes with error
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_NSObjectID_48_0 column1]: unrecognized selector sent to instance 0x5e22120'
Here if you notice, the value for key TableNameEntity is contained in quotes, so I guess its being returned as a string.
See if you can correct what I've done wrong.
I've tried an alternate to get columns values in the dictionary. Here it is (this works fine). But I guess its not a good approach.
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:subExp1, #"column1", #"column2", ... #"columnN" nil]];
You could make the calculation in a fetch request as follows:
- (NSArray *)myFetchResults
{
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:#"myEntity" inManagedObjectContext:myContext];
request.resultType = NSDictionaryResultType;
NSExpressionDescription *subExDescr = [[NSExpressionDescription alloc] init];
[subExDescr setName:#"subtraction"];
[subExDescr setExpression:[NSExpression expressionForFunction:#"subtract:from:"
arguments:[NSArray arrayWithObjects:
[NSExpression expressionForConstantValue:[NSNumber numberWithInt:someNumber]],
[NSExpression expressionForKeyPath:#"myAttribute"],
nil]]];
[subExDescr setExpressionResultType:NSInteger64AttributeType];
request.propertiesToFetch = [NSArray arrayWithObject:subExDescr, nil];
NSError *err = nil;
NSArray *results = [self.moContext executeFetchRequest:request error:&err];
[request release];
[err release];
return results;
}
The fetch results will be an array of dictionaries. You can access the result for the nth value in the column as follows:
NSArray *results = [self myFetchResults];
NSDictionary *nthDict = [results objectAtIndex:n];
NSInteger nthValue = [nthDict objectForKey:#"subtraction"];
Note that this code is untested. As is, I believe it will operate on all items in the column. If you want to operate only on selected items, you can add a predicate to select the items you want to operate on.
You could also look up the documentation for NSExpression and build all sorts of different operations. The class is a bit dense, but the Apple documentation has some snippets of code that help to understand how to use it. And I hope the above example illustrates how to incorporate it into a fetch request.
EDIT: CORRECTION
The entity, of course, has to be specified in the fetch request. I had initially left that out, but have now corrected the code.
EDIT: RESPONSE TO COMMENT
I'm not sure I understand what you're asking, but this may be it. You can create expression descriptions as follows:
NSExpressionDescription *expLatitude = [[NSExpressionDescription alloc] init];
[expLatitude setName:#"latitude"];
[expLatitude setExpression:[NSExpression expressionForKeyPath:#"Column3"]];
[expLatitude setExpressionResultType:NSDoubleAttributeType];
NSExpressionDescription *expEntity = [[NSExpressionDescription alloc] init];
[expEntity setName:#"TableNameEntity"];
[expEntity setExpression:[NSExpression expressionForKeyPath:#"objectID"]];
[expEntity setExpressionResultType:NSObjectIDAttributeType];}
Then, you add them to the propertiesToFetch array, as two more objects. Each dictionary in the fetch results will now have the latitude, the subtraction resulting from that same latitude, and the corresponding objectID of the entity that contained that latitude. The dictionaries will be ordered in the results array according to your sort descriptors. (I haven't tried the objectID expression, but I think it should work fine.)
Basically, your results are ordered in the exact same order as a traditional fetch request with the same predicate and the same sort descriptors, that is, for a fetch request with the default result type NSManagedObjectResultType.
I hope this answers your question. If not, don't hesitate to ask again. But, I may take a while to answer because it's sleep time for me now.
EDIT: RESPONSE TO 'GET COREDATA OBJECT'
Good catch on finding the correct expression to get the object ID! (Seeing it, the expression I offered for this now looks obviously wrong.)
As to the exception you're getting, it makes sense. The value returned in the fetch results is not the managed object itself, it's only the managed object's ID. To access the managed object, I think the following should work:
NSManagedObjectID *myObjectID = [dict objectForKey:#"TableNameEntity"];
TableNameEntity *tableEntity = (TableNameEntity *)[context objectWithID:myObjectID];
tableEntity.column1
The context above is the NSManagedObjectContext.
However, I think I prefer your final solution. I didn't know you could combine NSExpressionDescriptions with properties in the propertiesToFetch array. Good to know!
More importantly, it may be faster to get all the properties you need in the fetch than to get only the objectID from the fetch and get the properties later. Getting the objectID generally does not fire the fault for the entity. I believe the fault will be fired later, when you access the properties. It will fire once, when accessing the first property, or multiple times, once for each property. (I'm not sure which.) [For an explanation of faulting, see Firing Faults.]
My recommendation is that including all the properties you need in propertiesToFetch is the best approach. (You may try getting the objectID, if you prefer. But, if you find it's slow, you can go back to getting all the properties in the fetch.)
Fetch requests and expressions are poorly documented. You have to play with them a bit to get the syntax right. You seem to be doing very well.
I have implement a UItable with data sourced via Core Data. The table works fine and presents the data correctly, drills down.. etc.
However, it has the following problem: it presents the content data in a different order every time. I would like it at least to appear consistently or even better alphabetically.
Any ideas on why this might be happening or a specific property or method I should be reviewing in the docuemntation?
Help much appreciated
You need to set an NSSortDescriptor on your NSFetchRequest
NSSortDescriptor *sortDescriptorName = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES selector:#selector(caseInsensitiveCompare:)];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptorName]];
Then create your NSFetchedResultsController with the fetchRequest object.
This would cause your list to appear sorted alphabetically using the property "name" and ignore case while sorting.
You can simply add a sort descriptor to the request. In this example, the data object has a numeric column for "sortOrder", but you could sort on most anything.
- (NSMutableArray *)loadData {
NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"NameOfEntity" inManagedObjectContext:context];
[request setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"sortOrder" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[context executeFetchRequest:request error:&error] mutableCopy];
if (mutableFetchResults == nil) {
// Handle the error.
}
[request release];
return [mutableFetchResults autorelease];
}
I have not done much with CoreData other than to read about it and review some examples, but I would say this:
It sounds like your data is simply coming out of CoreData unsorted, and the order is in fact not guaranteed UNLESS you provide some additional hint about how you would like it sorted, with say a sort descriptor or something.
Sorry I can't provide a concrete reference, but there has to be a way to both fetch data and specify the order in which it is returned.
Alternatively, you could (behind the scenes) fetch all the table data from CoreData and then sort it yourself, but I think that defeats the purpose of using CoreData in the first place and discards a lot of the functionality of CoreData that is likely more efficient than anything you could write yourself to massage the data.
I have a core data entity called images that has just 2 fields:
imageName = NSString
timeStamp = NSNumber
I am trying to simulate a kind of stack LIFO (last in first out).
Inserting a new entry is easy but what about reading the last entry added to the entity?
All images are added with a timestamp, obtained by using
time_t unixTime = (time_t) [[NSDate date] timeIntervalSince1970];
an integer that is equal to the number of seconds since 1970
so, how do I retrieve the last inserted record of a core data (= the record that has the biggest timestamp number)???
thanks
Perform a fetch request, sorting the results by timeStamp.
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:...];
// Results should be in descending order of timeStamp.
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"timeStamp" ascending:NO];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
NSArray *results = [managedObjectContext executeFetchRequest:request error:NULL];
Entity *latestEntity = [results objectAtIndex:0];
You might also want to restrict the number of results using NSFetchRequest's setFetchLimit:.
I have tried using the method that Chris Doble mentioned and found it to be very slow, especially if there are lot of records that would need to be pulled and checked against the timeStamp. If you want to speed things up, I am now setting an attribute called isMostRecent on my ManagedObject's that I may ever want to get the most recent from. When a new record is to be stored I just grab the most recent record that has this attribute set to YES and change it to NO then set the new record that is being stored to YES. The next time I need to grab to most recent record all I have to do is this...
+ (Photo*)latestPhotoForMOC:(NSManagedObjectContext*)context {
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:kCoreDataEntityNamePhoto
inManagedObjectContext:context];
[request setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"isMostRecent == %#", [NSNumber numberWithBool:YES]];
[request setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"isMostRecent" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[context executeFetchRequest:request error:&error] mutableCopy];
Photo* photo = nil;
if (mutableFetchResults && mutableFetchResults.count > 0) {
photo = [mutableFetchResults objectAtIndex:0];
}
return photo;
}
I have found this to be much faster. Yes, it requires a little more on your part to ensure it is used properly and that you don't ever end up with more than one record marked as isMostRecent but for me this was the best option.
Hope this helps someone else too.
In Swift 4, declare:
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
let entity = [Entity]()
func getLastRecord() {
let entityCount = (entity.count - 1)
let lastRecord = entity[entityCount] // This is the las attribute of your core data entity
print(lastRecord)
}