Core data basics

core_data.m

Core Data Querying Basics
Jul 13, 2014

The basics of querying, filtering, sorting, and grouping persistent data in Core Data.

Retrieving Persistent Objects
// instances of the entity to fetch
NSEntityDescription *entity
  = [NSEntityDescription entityForName:@"Item"
                inManagedObjectContext:self.managedObjectContext];

/* `NSFetchRequest` used to query
   _must_ have an `entity` set */
NSFetchRequest *request = [[NSFetchRequest alloc] init];
// set entity to fetch
request.entity = entity;

// `NSManagedObjectContext` executes queries
NSError *e = nil;
NSArray *results
  = [self.managedObjectContext executeFetchRequest:request
                                             error:&e];
if (e) {
    NSLog(@"fetch error: %@", [e userInfo]);
    abort();
}

// output all persistent `Item` instances
for (NSManagedObject *item in results) {
    NSLog(@"item: %@", [item valueForKey:@"name"]);
}
Conditions
NSEntityDescription *entity
  = [NSEntityDescription entityForName:@"Item"
                inManagedObjectContext:self.managedObjectContext];

// `NSPredicate` used to filter
NSPredicate *predicate
  = [NSPredicate predicateWithFormat:@"name BEGINSWITH[c] %@", @"B"];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = entity;
// set filter condition
request.predicate = predicate;

NSError *e = nil;
NSArray *results
  = [self.managedObjectContext executeFetchRequest:request
                                             error:&e];
if (e) {
    NSLog(@"fetch error: %@", [e userInfo]);
    abort();
}

// output `Items` having a name starting with "B" (case-insensitive)
for (NSManagedObject *item in results) {
    NSLog(@"item: %@", [item valueForKey:@"name"]);
}
Sorting
NSEntityDescription *entity
  = [NSEntityDescription entityForName:@"Item"
                inManagedObjectContext:self.managedObjectContext];

/* `NSSortDescriptor` used to sort
   needs an attribute name and direction */
NSSortDescriptor *mostRecentFirst
  = [NSSortDescriptor sortDescriptorWithKey:@"createdAt"
                                  ascending:NO];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = entity;
// set sort condition
request.sortDescriptors = @[mostRecentFirst];

NSError *e = nil;
NSArray *results
  = [self.managedObjectContext executeFetchRequest:request
                                             error:&e];
if (e) {
    NSLog(@"fetch error: %@", [e userInfo]);
    abort();
}

// output `Items` ordered most recent first
for (NSManagedObject *item in results) {
    NSLog(@"item: %@, createdAt: %@",
      [item valueForKey:@"name"],
      [item valueForKey:@"createdAt"]);
}
Grouping and Aggregation
NSEntityDescription *entity
  = [NSEntityDescription entityForName:@"Item"
                inManagedObjectContext:self.managedObjectContext];

// attribute to group by
NSExpression *keyPath
  = [NSExpression expressionForKeyPath:@"name"];
// aggregate function (count, sum, avg, min, max, etc.)
NSExpression *expression
  = [NSExpression expressionForFunction:@"count:"
                              arguments:@[keyPath]];

/* `NSPropertyDescription` subclass used for fetching
   needs a name, expression, and expression type */
NSExpressionDescription *calc
  = [[NSExpressionDescription alloc] init];
calc.name = @"total";
calc.expression = expression;
calc.expressionResultType = NSInteger32AttributeType;

NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = entity;
// set grouping attribute
request.propertiesToGroupBy = @[@"name"];
// fetch only the group name and calculation
request.propertiesToFetch = @[@"name", calc];
// ask for dictionaries not managed objects
request.resultType = NSDictionaryResultType;

NSError *e = nil;
NSArray *results
  = [self.managedObjectContext executeFetchRequest:request
                                             error:&e];
if (e) {
    NSLog(@"fetch error: %@", [e userInfo]);
    abort();
}

// output each group and total
for (NSDictionary *d in results) {
    NSLog(@"name: %@, total: %@", d[@"name"], d[@"total"]);
}