Getting a maximum value from a core data relationship - ios

I search the net for answers to this problem, but unfortunately I don't think I am fluent enough in core data procedures to actually include the right combo of keywords.
I have two entities... Users and Bookmarks with a one-to-many relationship.
Users : string:firstName, string:lastname, string:iconImage
with a relationship to Bookmarks - inverse
Bookmarks : string:title, string:url, string:content, image:Binary Data, order:Integer32
with a relationship to Users - inverse
My goal here is to query the bookmarks for a particular user and find the highest number for the key:order, so that when I add an new bookmark, it will be one larger than the max. I have seen the Apple example, which works and makes sense, but I need a little more. The example returns the maximum value for all records in that entity.
-(NSNumber*) getNextBookmarksOrderForUser:(NSManagedObjectID*)userID
{
NSNumber *highOrder;
Users *user =[self getUserByID:userID];
if (user)
{
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"order"];
NSExpression *highestOrderingNumber = [NSExpression expressionForFunction:#"max:" arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc]init];
[expressionDescription setName:#"maxOrdering"];
[expressionDescription setExpression:highestOrderingNumber];
[expressionDescription setExpressionResultType:NSDecimalAttributeType];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Bookmarks" inManagedObjectContext:[_dataContext managedObjectContext]];
NSFetchRequest *request = [[NSFetchRequest alloc]init];
[request setEntity:entity];
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
[request setResultType:NSDictionaryResultType];
NSError *error = nil;
NSArray *objects = [[_dataContext managedObjectContext] executeFetchRequest:request error:&error];
if(objects == nil) {
// Handle the error
}
else {
if ([objects count] > 0) {
highOrder = [NSNumber numberWithInt:[[[objects objectAtIndex:0] valueForKey:#"maxOrdering"]intValue]];
NSLog(#"Highest ordering number: %#", highOrder);
}
}
return highOrder;
}
return nil;
}
So if I have two users, one with 10 bookmark and the other with 25, the above code will always return 25. I tried adding a predicate:
NSPredicate *byUser = [NSPredicate predicateWithFormat:#"self == %#", user];
[request setPredicate:byUser];
Does anyone have any suggestions? I Know that I could return all bookmark for a user and the write the code to find my value, but I would like to do it the most efficient way.
Thank you for any help.

From my comment
I think you should use a predicate like this [NSPredicate predicateWithFormat:#"users == %#", user]; Since you are querying against Bookmarks.

Related

Query max NSDate in core data

Is there a way to query NSDate in CoreData. For example if I want an entity with the highest NSDate value? I see that NSExpression "max:" only takes an NSNumber.
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"]);
}
}
more detail under Fetching Managed Objects -> Fetching Specific Values in the CoreData documentation.
or
Perfomed a query, ordered on Date field DESCENDING, and using setFetchLim it:1.
Its not perfect, but at least it worked.
You can do this directly in SQLite-- without fetching everything and then filtering the result, and without the complexity of NSExpression.
To get the one object that has the max date, do something like (assuming entity name Entity and date attribute timeStamp):
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:#"Event"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"timeStamp = self.#max.timeStamp"];
fetchRequest.predicate = predicate;
Do the fetch. You'll get (at most) one result, which will be the instance with the max date.
If you want to get just the date and not the entire managed object, add this before doing the fetch:
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.propertiesToFetch = #[ #"timeStamp" ];
you can get it with
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:#"tablename"];
fetchRequest.fetchLimit = 1;
fetchRequest.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"yourDate" ascending:NO]];
NSError *error = nil;
id person = [managedObjectContext executeFetchRequest:fetchRequest error:&error].firstObject;

Core Data - detect equal properties in fetched objects

My core data model:
Contact
=======
name
phone number (unique)
I need to display a table view of contacts with the following condition:
if there is more than one contact with the same name - show name+number
otherwise - show only name
Fo for example, if my core data contacts are:
Michael, 11112221
Jon, 33438282
Jon, 72727272
Lisa, 99939393
My table view should present:
Jon (33438282)
Jon (72727272)
Lisa
Michael
Currently I'm using the following NSFetchRequest to show the contact list:
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:#"Contact"];
request.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES selector:#selector(caseInsensitiveCompare:)]];
request.fetchBatchSize = 20;
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
managedObjectContext:context
sectionNameKeyPath:#"nameFirstLetter"
cacheName:nil];
I'm looking for the most efficient way to know which name appears more than once so I can show the number next to the name.
If you're using NSSQLiteStoreType as your persistent store you can fetch duplicate items with an NSExpression:
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Contact" inManagedObjectContext:self.managedObjectContext];
NSAttributeDescription *nameDesc = [entity.attributesByName objectForKey:#"name"];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"name"];
NSExpression *countExpression = [NSExpression expressionForFunction:#"count:" arguments:#[keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: #"count"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
NSError *error = nil;
NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:#"Contact"];
[fetch setPropertiesToFetch:#[nameDesc, expressionDescription]];
[fetch setPropertiesToGroupBy:#[nameDesc]];
[fetch setResultType:NSDictionaryResultType];
NSArray *results = [self.managedObjectContext executeFetchRequest:fetch error:&error];
NSArray *duplicates = [results filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:#"count > 1"]];
NSArray *duplicateNames = [duplicates valueForKeyPath:#"name"];
duplicateNames contains the name of contacts which name appears multiple times in the db. So, whenever a contact displayed in the table view, just query the duplicateNames array to check whether the actual contact is a duplicate or not.
The easiest way if you only want to modify the display I would modify tableView:cellForRowAt... to check the name of the person above and below the current one
in basic mock code:
- tableView:cellForRowAtIndexPath:(id)path {
....
Person *prior = ...
Person *next = ...
if(prior.name == current.name || next.name == current.name) {
//show name+number
}
}

Retrieving count of a core data relation

I searched high and low but I couldn't find exactly what I was looking for. My question is similar to this, but slightly different:
Core Data - Count of Related Records
Let's say I have a Car entity which has a one to many relation with a Person entity. This means that the car could have multiple people driving it, but each person drives only one car.
I want to be able to execute only one predicate wherein I could achieve the following:
All cars which are 'red'.
Return only the 'Year' and 'Color' attributes of the matching car.
Return a count of how many people are driving this car (i.e the size of the NSSet of People inside each resulting Car).
Is it possible to do all this with one query?
I know how to do this with multiple queries. I would just use setPropertiesToFetch and use a filtered predicate to achieve 1 and 2 above. I would then perform another count query (countForFetchRequest) on the Persons entity for every car to find how many Person(s) drive each car.
The key is the 3rd requirement above. I want to do everything in one predicate and I don't want to bring all of the Person entity objects into memory (performance) on the initial query. Furthermore it hurts to call another countForFetchRequest query for each car.
What's the best way to do this?
Thanks!
I cannot test this at the moment, but that should be possible by adding the following expression description to the "properties to fetch":
NSExpression *countExpression = [NSExpression expressionForFunction: #"count:" arguments: [NSArray arrayWithObject:[NSExpression expressionForKeyPath: #"drivers"]]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: #"driversCount"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
Return only 'red' cars:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"color LIKE 'red'"];
Return a count of how many people are driving this car:
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"people"];
NSExpression *countExpression = [NSExpression expressionForFunction:#"count:"
arguments:#[keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:#"count"];
[expressionDescription setExpression:countExpression];
[expressionDescription setExpressionResultType:NSInteger32AttributeType];
Return only the 'year' and 'color' attributes (and the count):
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Car"
inManagedObjectContext:context];
NSDictionary *attributes = [entity attributesByName];
NSArray *properties = #[expressionDescription, attributes[#"year"], attributes[#"color"]];
Build and execute the fetch request:
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setResultType:NSDictionaryResultType];
[request setPropertiesToFetch:properties]; // return only count, year & color
[request setPredicate:predicate]; // return only red cars
NSError *error = nil;
NSArray *results = [context executeFetchRequest:request error:&error];
Process the results:
if (results) {
for (NSDictionary *result in results) {
NSLog(#"Year: %#", result[#"year"]);
NSLog(#"Color: %#", result[#"color"]);
NSLog(#"Drivers: %#", result[#"count"]);
}
}
else {
NSLog(#"Error: %#", error);
}

Core Data Queries

I am still trying to wrap my head around Core Data and am having trouble letting go of SQL. I know that they are not the same but I am finding it hard to recreate what I can do in SQL. I am hoping someone can help.
What I want to do is create a list of Clients and return a sum of their current bill. I have two entities:
Client
name
address
etc
Billing
Client
date
amount
etc
I have created a one-to-many relationship between the 2 entities and have my NSEntityDescription pointing to Client.
Now what I want to happen in SQL terms is this:
SELECT *, SUM(amount) as Total FROM Clients INNER JOIN Billing......"
Can this be done with one Fetch Request or would I do 2 Requests and then merge them?
Can I somehow use valueForKey:#"billing.#sum.amount"?
Also could anyone recommend a great Core Data book (or website) for beginners, intermediate level?
Can this be done with one Fetch Request or would I do 2 Requests and then merge them?
Yes it can be done with one request. see here
/*UNTESTED*/
- (NSNumber*) billingSumForClient:(NSManagedObjectID*)clientId
context:(NSManagedObjectContext*)context
{
NSNumber* total = nil;
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:#"Billing"];
[request setResultType:NSDictionaryResultType];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:#"amount"];
NSExpression *sumExpression = [NSExpression expressionForFunction:#"sum:"
arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:#"total"];
[expressionDescription setExpression:sumExpression];
[expressionDescription setExpressionResultType:NSDateAttributeType];
[request setPredicate:[NSPredicate predicateWithFormat:#"client == %#",clientId]];
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
NSError *error = nil;
NSArray *objects = [context executeFetchRequest:request error:&error];
if (&error) {
// Handle the error.
} else {
if ([objects count] > 0) {
total = [[objects objectAtIndex:0] valueForKey:#"total"];
}
}
return total;
}
Can I somehow use valueForKey:#"billing.#sum.amount"?
Yes. see here(#Daniel link)
This might be faster if all your billings are already faulted into your current context.

How do you construct the predicate for NSFetchRequest setHavingPredicate:?

For the life of me I can not seem to get this to work.
Assume our entity is an managed object with a status field and an order field.
How would I go about getting all orderedEntries having more than one order that are the same?
Please no answers telling me to just do a subquery with #count in the main predicate, since I know of that solution, the point of this post is to understand how to use the having predicate in core data, which would probably be faster than a subquery anyways. (unless you explain why I can not use a having clause)
The following code would return an array of dictionaries with the number of orders per order number. What I want is to be able to add a having clause to restrict my request to only return the dictionaries representing objects of those orders that have a count greater than 1.
Here is the code so far and my attempts at a having predicate:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"OrderedEntry"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];
[fetchRequest setResultType:NSDictionaryResultType];
NSPredicate *predicate = [NSPredicate predicateWithFormat:
#"(status == %#)",[NSNumber numberWithInt:EntryStatusAlive]];
[fetchRequest setPredicate:predicate];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: #"order"]; // Does not really matter
NSExpression *maxExpression = [NSExpression expressionForFunction: #"count:"
arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: #"orderCount"];
[expressionDescription setExpression: maxExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
[fetchRequest setPropertiesToFetch: [NSArray arrayWithObjects:expressionDescription,#"order",nil]];
[fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObjects:#"order",nil]];
//[fetchRequest setHavingPredicate:[NSPredicate predicateWithFormat:#"#count > 1"]];
//[fetchRequest setHavingPredicate:[NSComparisonPredicate predicateWithLeftExpression:maxExpression rightExpression:[NSExpression expressionForConstantValue:[NSNumber numberWithInteger:1]] modifier:NSDirectPredicateModifier type:NSGreaterThanPredicateOperatorType options:NSCaseInsensitivePredicateOption]];
NSError *error;
NSArray * array = [context executeFetchRequest:fetchRequest error:&error];
I ended up going with this for anyone interested
-(BOOL)ordersAreSaneOnDay:(NSNumber*)dayNumber forUser:(User*)user inContext:(NSManagedObjectContext*)context {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"BasicEntry"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];
[fetchRequest setResultType:NSDictionaryResultType];
NSPredicate *predicate = [NSPredicate predicateWithFormat:
#"(status == %#) && ((type != %#) && (type != %#) && (dayNumber == %#)) && ((user == NIL) || (user == %#))",[NSNumber numberWithInt:EntryStatusAlive],[NSNumber numberWithInt:EntryTypeTask],[NSNumber numberWithInt:EntryTypeCompletedTask],dayNumber,user];
[fetchRequest setPredicate:predicate];
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath: #"order"]; // Does not really matter
NSExpression *maxExpression = [NSExpression expressionForFunction: #"count:"
arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName: #"orderCount"];
[expressionDescription setExpression: maxExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];
[fetchRequest setPropertiesToFetch: [NSArray arrayWithObjects:expressionDescription,#"order",nil]];
[expressionDescription release];
[fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObjects:#"order",nil]];
//[fetchRequest setHavingPredicate:[NSPredicate predicateWithFormat:#"self.order.#count > 1"]];
//[fetchRequest setHavingPredicate:[NSComparisonPredicate predicateWithLeftExpression:maxExpression rightExpression:[NSExpression expressionForConstantValue:[NSNumber numberWithInteger:1]] modifier:NSDirectPredicateModifier type:NSGreaterThanPredicateOperatorType options:NSCaseInsensitivePredicateOption]];
NSError *error;
NSArray * array = [context executeFetchRequest:fetchRequest error:&error];
array = [array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:#"orderCount > 1"]];
//NSLog(#"it worked %#",array);
[fetchRequest release];
if ([array count]) return FALSE;
return TRUE;
}
I've got this working using the following:
[fetchRequest setHavingPredicate:[NSPredicate predicateWithFormat:#"$orderCount > 1"]];
Use the name of your expressionDecription as variable $orderCount.
Alternatively you can use
NSExpression *countExpression = [NSExpression expressionForVariable:#"orderCount"];
[fetchRequest setHavingPredicate:[NSPredicate predicateWithFormat:#"%# > 1", countExpression]];
Firstly, whenever I try something analogous to what you're doing, I get an error that you can't pass a to-many relationship to setPropertiesToFetch:. The NSFetchRequest documentation backs this up: "The property descriptions may represent attributes, to-one relationships, or expressions." So that's problem #1.
Problem #2 is that it appears that you can't group by a to-many relationship either (this isn't made clear in the documentation, but you get the same error and it also makes sense).
Remove "order" from the properties to fetch. Group by an attribute. Modify your main predicate to only include those attributes you group by (or just remove it). Specify "order" in your having predicate.
[fetchRequest setPropertiesToGroupBy: #[ #"???" ]];
[fetchRequest setHavingPredicate: [NSPredicate predicateWithFormat: #"self.order.#count > 1"]];
Now you'll see the request will work, but the results probably weren't what you were expecting:
- NSArray
- NSDictionary { status = #"alive", orderCount = "4" }
- NSDictionary { status = #"alive", orderCount = "9" }
- NSDictionary { status = #"alive", orderCount = "2" }
- etc...
NSDictionaryResultType doesn't actually give you anything to identify those objects by - it just gives you the values.
So your next step is to get back IDs for those OrderedEntry objects. The trick is to include an expression which will give you back the NSManagedObjectID as a key in each dictionary.
I don't know if this will actually give you improved performance at all (over just AND-ing it in to the main predicate). In my experience, one of the best things you can do to improve fetching performance is to create singleton NSPredicates and use substitution variables to set up each fetch. Predicate parsing can be expensive.
Dictionary result types can be a pain. Usually just constructing a good predicate is better. I tend only to use them for expressions (i.e. performing statistic-type calculations on the graph which return a value). If you look at all the restrictions around properties to fetch and group by and the predicate restrictions, this seems to be what Apple intend it for. I'm not even sure it's possible to do what you want to do by grouping and using a having predicate - for example, what are you going to group by? If by status (which you need to group by to include in your predicate), won't that collapse all the OrderEntries and you won't get the separate objectIDs and orderCounts? It's best to stick to the main predicate for this, not grouping and having predicates.

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