Correct way to modify results array from NSFectchRequest without changing Core Data objects - iphone

Question

I am retrieving a fetch from my core data database and trying to iterate through the data and make changes to the data, if necessary. When I change the data in the results array, it turns out that my database is changing in the back end as well, without doing a save. I am wondering what would be a good practice to use to change the data without affecting the back end data.
Here is the code I change the data with:
self.singleDayDataPointsForGraph = [[self fuelPurchaseDataForTimePeriodInMonths:self.numberOfMonthsForGraphView] mutableCopy];
for (int i = 0; i < self.singleDayDataPointsForGraph.count; i++) {
FuelPurchase *currentFuelPurchase = [self.singleDayDataPointsForGraph objectAtIndex:i];
if (i < self.singleDayDataPointsForGraph.count + 1 && self.singleDayDataPointsForGraph.count >= 2) {
FuelPurchase *purchaseToCompare = [self.singleDayDataPointsForGraph objectAtIndex:i + 1];
NSDate *firstDate = currentFuelPurchase.dateTimeStamp;
NSDate *secondDate = purchaseToCompare.dateTimeStamp;
NSDateFormatter *dateComparisonFormatter = [[NSDateFormatter alloc] init];
[dateComparisonFormatter setDateFormat:#"yyyy-MM-dd"];
if([[dateComparisonFormatter stringFromDate:firstDate] isEqualToString:[dateComparisonFormatter stringFromDate:secondDate]] ) {
float firstValue = [purchaseToCompare.fillSavings floatValue];
float secondValue = [currentFuelPurchase.fillSavings floatValue];
purchaseToCompare.fillSavings = [NSNumber numberWithFloat:(firstValue + secondValue)];
[self.singleDayDataPointsForGraph removeObjectAtIndex:i];
}
}
The fuelPurchaseDataForTimePeriodInMonths: method is what performs the fetch and returns an NSArray of results. self. singleDayDataPointsForGraph is an NSMutableArray that stores the results array as a mutable copy. This method basically checks two entries in the database to see if they are the on the same day and if they are, then it adds the fuel purchase amounts to each other and deletes one of the records. I don't want this to change my back end data, but it is.
Thanks very much.

Answer 1

It is doing the right thing. If you take a core data object and modify it, it will reflect immediately whether you save it ot not. The saving part ensures, that if you quit the application and come back the data is saved as well.
So for your situation, I would avoid modifying the actual core data object. Rather create a structure which imitates the core data object and modify that structure.
Example, say my coredata object is Person with attributes name and age.
The object a get from a fetch is say
person1.
You have a class PersonSub with same attributes.
Now you can create
PersonSub *personSub = [[PersonSub alloc] init]; //You can create a custom init to initilize from Person core data if you like.
personSub.name = person1.name;
personSub.age = person1.age;
Now you can modify as follows
personSub.age = personSub.age + 1;