I need to get the latest date from coredata
i found a way
NSSortDescriptor * sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"date" ascending:NO];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
[fetchRequest setFetchLimit:1];
so sort them by date and then pick the first
however can this not be done more optimal? this approach looks like a brute force
sort is nlogn, but simple search for the max is n
You can actually ask SQL for just that value, not the object with that value:
NSExpression *date = [NSExpression expressionForKeyPath:#"date"];
NSExpression *maxDate = [NSExpression expressionForFunction:#"max:"
arguments:[NSArray arrayWithObject:maxDate]];
NSExpressionDescription *d = [[[NSExpressionDescription alloc] init] autorelease];
[d setName:#"maxDate"];
[d setExpression:maxSalaryExpression];
[d setExpressionResultType:NSDateAttributeType];
[request setPropertiesToFetch:[NSArray arrayWithObject:d]];
NSError *error = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
// Handle the error.
} else {
if (0 < [objects count]) {
NSLog(#"Maximum date: %#", [[objects objectAtIndex:0] valueForKey:#"maxDate"]);
}
}
This is described in more detail under Fetching Managed Objects -> Fetching Specific Values in the CoreData documentation.
On line 2 maxDate expression refers to itself (maxDate).
I assume this must be the "date" variable from the first line.
I guess that's pretty much the best way. I'm not sure whether there is a more efficient way since it has to compare every date anyway to figure out which the oldest is.
Here are 2 other ways:
1) You could work with BOOLs as an attribute of the managed object. (like oldest = 1)
However, you'd have to find a new "oldest" managed object every time you delete one.
2) You could just save the oldest one until it changes. This might save a lot of work if you have to find the oldest managedObject often.
It depends on your application (how many times you insert/remove managed objects and how many times you need the oldest object).
Related
I have a subclass of the CoreDataTableViewController (subclass of UITAbleViewController dome by the people on Stanford done to link CoreData and TableViews). On this Class, I want to perform a fecth, sorting by an attribute called "definition" and the code which executes it is the following:
- (void)setupFetchedResultsController{
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:self.entity];
request.propertiesToFetch=[NSArray arrayWithObject:#"definition"];
request.returnsDistinctResults=YES;
NSPredicate *predicate1 = [NSPredicate predicateWithFormat:#"%K != nil", #"definition"];
NSPredicate *predicate2 = [NSPredicate predicateWithFormat:#"%K != ''", #"definition"];
NSPredicate *predicate3= [NSPredicate predicateWithFormat:#"%K contains[cd] %#", #"definition", self.seachBar.text];
NSArray *prepredicateArray;
if ([self.seachBar.text length]) {
prepredicateArray = [NSArray arrayWithObjects:predicate1, predicate2, predicate3,nil];
}else {
prepredicateArray = [NSArray arrayWithObjects:predicate1, predicate2,nil];
}
request.predicate=[NSCompoundPredicate andPredicateWithSubpredicates:prepredicateArray];
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"definition" ascending:YES ]];
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
managedObjectContext:self.managedObjectContext
sectionNameKeyPath:nil
cacheName:nil];
[self performFetch];
}
If I understood it correctly, setting request.returnsDistinctResults=YES; should avoid fetching duplicates. However it doesn't work and I'm seeing duplicates of this attribute's value.
Is there something I'm missing there? I'd appreciate some pointings there. Thank you in advance.
EDIT: If anyone is having the same issue here, after applying David's answer the resulting fetchedResultsController is just a NSDIctionary with object with only the requested value, which for displaying only purposes is quite fine. One thing I've done in cellForRowAtIndexPath in order to display the results on the cell label is:
Before:
HNMR *hnmr = [self.fetchedResultsController objectAtIndexPath:indexPath];
cell.textLabel.text=hnmr.definition;
After:
cell.textLabel.text=[[self.fetchedResultsController objectAtIndexPath:indexPath] valueForKey:#"definition"];
From the documentation of returnsDistinctResults:
This value is only used if a value has been set for propertiesToFetch.
From the documentation of propertiesToFetch:
This value is only used if resultType is set to NSDictionaryResultType.
From the documentation of resultType:
The default value is NSManagedObjectResultType.
This all tells me that the propertiesToFetch is ignored because you haven't set the resultType yourself and the default it to return managed objects instead of dictionaries. Since the propertiesToFetch is ignored the returnsDistinctResults is ignored as well and thus you are still getting duplicates.
Try setting the result type to return dictionaries instead of managed objects.
request.resultType = NSDictionaryResultType;
In addition to David Rönnqvist answer I suggest a useful link (with a sample) on selecting distinct values with Core Data:
core-data-how-to-do-a-select-distinct
Hope that helps.
i have big pool of core date objects (around 10000) and there is too long time doing code according profile:
NSDate *maxExternalChangedDate = [codes valueForKeyPath:#"#max.externalChangedDate"];
is community know better way to found it?
NSString *rateSheetID = [rateSheetAndPrefix valueForKey:#"rateSheetID"];
NSFetchRequest *requestCodesForSale = [[NSFetchRequest alloc] init];
[requestCodesForSale setEntity:[NSEntityDescription entityForName:#"CodesvsDestinationsList"
inManagedObjectContext:self.moc]];
[requestCodesForSale setPredicate:[NSPredicate predicateWithFormat:#"(%K.carrier.GUID == %#)",relationshipName,carrierGUID]];
NSError *error = nil;
NSArray *codes = [self.moc executeFetchRequest:requestCodesForSale error:&error];
if (error) NSLog(#"Failed to executeFetchRequest to data store: %# in function:%#", [error localizedDescription],NSStringFromSelector(_cmd));
NSNumber *count = [codes valueForKeyPath:#"#count.externalChangedDate"];
if (count == 0) {
[requestCodesForSale release];
[pool drain], pool = nil;
return YES;
}
NSDate *maxExternalChangedDate = [codes valueForKeyPath:#"#max.externalChangedDate"];
By using NSFetchRequest and returning NSDictionaryResultType You can use NSExpressionDescription to yeild the results for functions like max() and min().
Sample Code from Apple
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"salary"];
NSExpression *maxSalaryExpression = [NSExpression expressionForFunction:#"max:"
arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:#"maxSalary"];
[expressionDescription setExpression:maxSalaryExpression];
[expressionDescription setExpressionResultType:NSDecimalAttributeType];
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
Check out this doc for more information.
Core Data Programming Guide
I have the same issue.
In theory this should work, but for me it did not.
For some reasons the query crashes with the error that the database is corrupt.
In the end I perfomed a query, where I ordered on my field DESCENDING, and using setFetchLim it:1. Its not perfect, but at least it worked.
Also I made sure the field I use has an index.
Since I have few records, this works fine.
On 30000 records, it might be a problem though.
I followed the IOS documentation, fot fatch a "max:" query, but only got "database corrupted" errors. That is the sample code from Apple fails BADLY.
Googling the internet, it seem the call to setPropertiesToFetch fails in IOS 5+ ??!
I have not found any way around that.
Using a normal query, it worked without any issue.
So I must conclude Apple code is no longer corerct.
I need to see if an object that I have obtained from a CSV file with a unique identifier exists in my Core Data Database, and this is the code I deemed suitable for this task:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity;
entity =
[NSEntityDescription entityForName:#"ICD9"
inManagedObjectContext:passedContext];
[fetchRequest setEntity:entity];
NSPredicate *pred = [NSPredicate predicateWithFormat:#"uniqueID like %#", uniqueIdentifier];
[fetchRequest setPredicate:pred];
NSError *err;
NSArray* icd9s = [passedContext executeFetchRequest:fetchRequest error:&err];
[fetchRequest release];
if ([icd9s count] > 0) {
for (int i = 0; i < [icd9s count]; i++) {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];
NSString *name = [[icd9s objectAtIndex:i] valueForKey:#"uniqueID"];
if ([name caseInsensitiveCompare:uniqueIdentifier] == NSOrderedSame && name != nil)
{
[pool release];
return [icd9s objectAtIndex:i];
}
[pool release];
}
}
return nil;
After more thorough testing it appears that this code is responsible for a huge amount of leaking in the app I'm writing (it crashes on a 3GS before making it 20 percent through the 1459 items). I feel like this isn't the most efficient way to do this, any suggestions for a more memory efficient way? Thanks in advance!
Don't use the like operator in your request predicate. Use =. That should be much faster.
You can specify the case insensitivity of the search via the predicate, using the [c] modifier.
It's not necessary to create and destroy an NSAutoreleasePool on each iteration of your loop. In fact, it's probably not needed at all.
You don't need to do any of the checking inside the for() loop. You're duplicating the work of your predicate.
So I would change your code to be:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:...];
[fetchRequest setPredicate:[NSPredicate predicateWithFormat:#"uniqueID =[c] %#", uniqueIdentifier]];
NSError *err = nil;
NSArray *icd9s = [passedContext executeFetchRequest:fetchRequest error:&err];
[fetchRequest release];
if (error == nil && [icd9s count] > 0) {
return [icd9s objectAtIndex:0]; //we know the uniqueID matches, because of the predicate
}
return nil;
Use the Leaks template in Instruments to hunt down the leak(s). Your current code may be just fine once you fix them. The leak(s) may even be somewhere other than code.
Other problems:
Using fast enumeration will make the loop over the array (1) faster and (2) much easier to read.
Don't send release to an autorelease pool. If you ever port the code to garbage-collected Cocoa, the pool will not do anything. Instead, send it drain; in retain-release Cocoa and in Cocoa Touch, this works the same as release, and in garbage-collected Cocoa, it pokes the garbage collector, which is the closest equivalent in GC-land to draining the pool.
Don't repeat yourself. You currently have two [pool release]; lines for one pool, which gets every experienced Cocoa and Cocoa Touch programmer really worried. Store the result of your tests upon the name in a Boolean variable, then drain the pool before the condition, then conditionally return the object.
Be careful with variable types. -[NSArray count] returns and -[NSArray objectAtIndex:] takes an NSUInteger, not an int. Try to keep all your types matching at all times. (Switching to fast enumeration will, of course, solve this instance of this problem in a different way.)
Don't hide releases. I almost accused you of leaking the fetch request, then noticed that you'd buried it in the middle of the code. Make your releases prominent so that you're less likely to accidentally add redundant (i.e., crash-inducing) ones.
I'm creating an iphone/ipad app that basically reads XML documents and creates tableviews from objects created based on the xml. The xml represents a 'level' in a filesystem. Its basically a browser.
Each time i parse the xml documents i update the filesystem which is mirrored in a core-data sqllite database. For each "File" encountered in the xml i attempt to get the NSManagedObject associated with it.
The problem is this function which i use to get/create either a new blank entity or get the existing one from database.
+(File*)getOrCreateFile:(NSString*)remotePath
context:(NSManagedObjectContext*)context
{
struct timeval start,end,res;
gettimeofday(&start,NULL);
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"File" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
[fetchRequest setFetchLimit:1];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"remotePath == %#",remotePath];
[fetchRequest setPredicate:predicate];
NSError *error;
NSArray *items = [context executeFetchRequest:fetchRequest error:&error];
[fetchRequest release];
File *f;
if ([items count] == 0)
f = (File*)[NSEntityDescription insertNewObjectForEntityForName:#"File" inManagedObjectContext:context];
else
f = (File*)[items objectAtIndex:0];
gettimeofday(&end, NULL);
[JFS timeval_subtract:&res x:&end y:&start];
count++;
elapsed += res.tv_usec;
return f;
}
For eksample, if i'm parsing a document with 200ish files the total time on a iPhone 3G is about 4 seconds. 3 of those seconds are spent in this function getting the objets from core data.
RemotePath is a unique string of variable length and indexed in the sqllite database.
What am i doing wrong here? or.. what could i do better/different to improve performance.
Executing fetches is somewhat expensive in Core Data, though the Core Data engineers have done some amazing work to keep this hit minimal. Thus, you may be able to improve things slightly by running a fetch to return multiple items at once. For example, batch the remotePaths and fetch with a predicate such as
[NSPredicate predicateWithFormat:#"remotePath IN %#", paths];
where paths is a collection of possible paths.
From the results, you can do the searches in-memory to determine if a particular path is present.
Fundamentally, however, doing fetches against strings (even if indexed) is an expensive operation. There may not be much you can do. Consider fetching against non-string attributes, perhaps by hasing the path and saving the hash in the entity as well. You'll get back a (potentially) larger result set which you could then search in memory for string equality.
Finally, do not make any changes without some performance data. Profile, profile, profile.
I have an method that reads an xml file and stores the xml nodes at a certain XPath-path in an NSArray called *nodes. What I want to do is take each one of the items in the array and add it to a core data entity called Category with the attribute of "name".
I have tried a number of different ways of creating the entity but I'm not sure about the correct way to do this effectively. This is the code used to create the NSArray, any ideas on how to implement this? (ignore the NSError, I will fix this in the final version)
- (IBAction)readCategories:(id)sender
{
NSString *xmlString = [resultView string];
NSData *xmlData = [xmlString dataUsingEncoding: NSASCIIStringEncoding];
NSXMLDocument *xmlDoc = [[NSXMLDocument alloc] initWithData:xmlData options:nil error:nil];
//XPath
NSError *err=nil;
NSArray *nodes = [xmlDoc nodesForXPath:#"//member[name='description']/value/string" error:&err];
}
EDIT - My loop code
NSArray *nodes = [xmlDoc nodesForXPath:#"//member[name='description']/value/string" error:&err];
int arrayCount = [nodes count];
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSXMLElement *categoryEl;
NSString *new = [catArrayController newObject];
int i;
for (i = 0; i < arrayCount; i++)
{
[categoryEl = [nodes objectAtIndex:i]];
[new setValue:[categoryEl stringValue] forKey:#"name"];
[catArrayController addObject:new];
}
[pool release];
Here's how I'd write it:
for (NSXMLElement *categoryElement in nodes) {
NSManagedObject *newObject = [catArrayController newObject];
[newObject setValue:[categoryElement stringValue] forKey:#"name"];
[catArrayController addObject:newObject];
[newObject release];
}
First, I'm using the Objective-C 2.0 for-each syntax. This is simpler than using index variables. I eliminated i and arrayCount.
Next, I took out your NSAutoreleasePool. None of the objects in the loop are autoreleased, so it had no effect. (The newObject method returns a retained object which is, by convention, what methods with the word new in their name do) This is also why I release newObject after adding it to the array controller. Since I'm not going to be using it any more in this method, I need to release it.
Also, you had defined new (which I renamed newObject) as an NSString. Core Data objects are always either an instance of NSManagedObject or a subclass of NSManagedObject.
Your line [categoryEl = [nodes objectAtIndex:i]] won't compile. That's because the bracket syntax is used to send a message to an object. This is an assignment statement, so the bracket syntax is not needed here. (This line is also not necessary any more because of I've changed the loop to use the for-each syntax) But, for future reference, categoryEl = [nodes objectAtIndex:i]; would have worked.
What part are you having trouble with? There shouldn't be much more to it than looping through the array, creating a new managed object for each entry, and setting the correct attributes. You can create the managed object with NSEntityDescription's -insertNewObjectForEntityForName:inManagedObjectContext: method.