Core Data Snippets

gistfile1.m

//1 Creating a New Managed Object Context
//To create a new managed object context, you need a persistent store coordinator.
NSPersistentStoreCoordinator *psc = <#Get the coordinator#>;
NSManagedObjectContext *newContext = [[NSManagedObjectContext alloc] init];
[newContext setPersistentStoreCoordinator:psc];

//using the same coordinator as the existing one
NSManagedObjectContext *context = <#Get the context#>;
NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator];
NSManagedObjectContext *newContext = [[NSManagedObjectContext alloc] init];
[newContext setPersistentStoreCoordinator:psc];

//2 Getting the Managed Object Model and Entities
NSManagedObjectContext *context = <#Get the context#>;
NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator];
NSManagedObjectModel *model = [psc managedObjectModel];
NSEntityDescription *entity = [[model entitiesByName] objectForKey:@"<#Entity name#>"];

//3 Adding a Persistent Store
NSPersistentStoreCoordinator *psc = <#Get the coordinator#>;
NSURL *storeUrl = [NSURL fileURLWithPath:@"<#Path to store#>"];
NSString *storeType = <#Store type#>; // A store type, such as NSSQLiteStoreType
NSError *error;
if (![psc addPersistentStoreWithType:storeType configuration:nil
    URL:storeUrl options:nil error:&error]) {
    // Handle the error.
}

//4 Basic Fetch
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>"
    inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil) {
    // Handle the error.
}

//5 Fetch with Sorting
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>"
    inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"<#Sort key#>"
    ascending:YES];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
 
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil) {
    // Handle the error.
}

//6 Fetch with a Predicate
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>"
    inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"<#Predicate string#>",
    <#Predicate arguments#>];
[fetchRequest setPredicate:predicate];
 
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil) {
    // Handle the error.
}

//7 Fetch with a Predicate Template
// Assume an instance variable:
// NSPredicate *predicateTemplate;
 
- (NSPredicate *)predicateTemplate {
    if (predicateTemplate == nil) {
        predicateTemplate = [NSPredicate predicateWithFormat:@"<#Key#> <#Operator#> <#$Variable#>"];
    }
    return predicateTemplate;
}

// use predicateTemplate
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>"
    inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 
NSDictionary *variables = @{ @"<#Variable#>" : <#Value#> };
NSPredicate *predicate = [[self predicateTemplate]
    predicateWithSubstitutionVariables:variables];
[fetchRequest setPredicate:predicate];
 
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil) {
    // Handle the error.
}

//8 Fetch with Sorting and a Predicate
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
 
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"<#Sort key#>" ascending:YES];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
 
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"<#Predicate string#>",
    <#Predicate arguments#>];
[fetchRequest setPredicate:predicate];
 
NSError *error;
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil) {
    // Handle the error.
}

//9 Fetching Distinct Values
NSManagedObjectContext *context = <#Get the context#>;
 
NSEntityDescription *entity = [NSEntityDescription  entityForName:@"<#Entity name#>" inManagedObjectContext:context];
 
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setResultType:NSDictionaryResultType];
[request setReturnsDistinctResults:YES];
[request setPropertiesToFetch:@[@"<#Attribute name#>"]];
 
// Execute the fetch.
NSError *error;
id requestedValue = nil;
NSArray *objects = [context executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}

//10 Fetching Attribute Values that Satisfy a Given Function
NSManagedObjectContext *context = <#Get the context#>;
 
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>" inManagedObjectContext:context];
[request setEntity:entity];
 
// Specify that the request should return dictionaries.
[request setResultType:NSDictionaryResultType];
 
// Create an expression for the key path.
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"<#Key-path for the property#>"];
 
// Create an expression to represent the function you want to apply
NSExpression *expression = [NSExpression expressionForFunction:@"<#Function name#>"
    arguments:@[keyPathExpression]];
 
// Create an expression description using the minExpression and returning a date.
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
 
// The name is the key that will be used in the dictionary for the return value.
[expressionDescription setName:@"<#Dictionary key#>"];
[expressionDescription setExpression:expression];
[expressionDescription setExpressionResultType:<#Result type#>]; // For example, NSDateAttributeType
 
// Set the request's properties to fetch just the property represented by the expressions.
[request setPropertiesToFetch:@[expressionDescription]];
 
// Execute the fetch.
NSError *error;
id requestedValue = nil;
NSArray *objects = [context executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}
else {
    if ([objects count] > 0) {
        requestedValue = [[objects objectAtIndex:0] valueForKey:@"<#Dictionary key#>"];
    }
}

//11 Creating a Managed Object
NSManagedObjectContext *context = <#Get the context#>;
<#Managed Object Class#> *newObject = [NSEntityDescription
    insertNewObjectForEntityForName:@"<#Entity name#>"
    inManagedObjectContext:context];
    
//12 Saving a Managed Object
NSManagedObjectContext *context = <#Get the context#>;
NSError *error;
if (![context save:&error]) {
    // Handle the error.
}

//13 Deleting a Managed Object
NSManagedObject *aManagedObject = <#Get the managed object#>;
NSManagedObjectContext *context = [aManagedObject managedObjectContext];
[context deleteObject:aManagedObject];
NSError *error;
if (![context save:&error]) {
    // Handle the error.
}