Core Data Snippets

Core Data Snippets Contents Introduction 3 Organization of This Document 3 Accessing the Core Data Stack 4 Core Data Stack Architectures 4 OS X 4 iOS 4 Getting a Managed Object Context 4 Creating a New Managed Object Context 5 Getting the Managed Object Model and Entities 6 Adding a Persistent Store 7 Fetching Managed Objects 8 Basic Fetch 8 Fetch with Sorting 8 Fetch with a Predicate 9 Fetch with a Predicate Template 10 Fetch with Sorting and a Predicate 11 Fetching Specific Property Values 13 Fetching Distinct Values 13 Fetching Attribute Values that Satisfy a Given Function 14 Creating and Deleting Managed Objects 16 Creating a Managed Object 16 Saving a Managed Object 16 Deleting a Managed Object 17 Document Revision History 18 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 2 This document contains prototypes for commonly-used snippets of code that you’re likely to use in a program that uses Core Data. In some cases (particularly in cases where a code snippet might be only one or two lines), the document provides guidance rather than explicit code. This document does not provide an in-depth explanation of the code snippets. You’re expected to be familiar with the Core Data framework and understand how to use the snippets in context. To learn more about Core Data, read Core Data Starting Point . Organization of This Document The document contains the following articles: ● “Accessing the Core Data Stack” (page 4) ● “Fetching Managed Objects” (page 8) ● “Fetching Specific Property Values” (page 13) ● “Creating and Deleting Managed Objects” (page 16) 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 3 Introduction This article contains snippets for creating and accessing parts of the main pieces of infrastructure defined by the Core Data framework. Core Data Stack Architectures How you access the parts of the Core Data stack may depend in part on the application architecture and platform. OS X Broadly-speaking, in OS X there are two basic application architectures for programs that use Core Data: ● Single-coordinator applications. These applications typically have a single Core Data stack (as defined by a single persistent store coordinator) managed by a single controller object. They generally use a single persistent store for the whole application. ● Document-based applications. These applications typically use the Application Kit’s NSPersistentDocument class. There is a usually a persistent store coordinator and a single persistent store associated with each document. This article uses the terms “single-coordinator application” and “document-based application” to differentiate between these architectures. iOS In iOS, the application delegate usually maintains a persistent store coordinator that manages the application’s store. It typically creates a managed object context, but it often doesn’t own it. This is explained in greater detail in “Getting a Managed Object Context” (page 4). Getting a Managed Object Context In OS X: 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 4 Accessing the Core Data Stack ● In an single-coordinator applications, you can get the application’s context directly from the application delegate. ● In document-based applications, you can get the context directly from the document instance. In iOS: ● By convention , you get a context from a view controller. You must implement your application appropriately, though, to follow this pattern. When you implement a view controller that integrates with Core Data, you can add an NSManagedObjectContext property. When you create a view controller, you pass it the context it should use. You pass an existing context, or (in a situation where you want the new controller to manage a discrete set of edits) a new context that you create for it. It’s typically the responsibility of the application delegate to create a context to pass to the first view controller that’s displayed. A view controller typically shouldn’t retrieve the context from a global object such as the application delegate—this makes the application architecture rigid. Neither should a view controller create a context for its own use (unless it’s a nested context). This may mean that operations performed using the controller’s context aren’t registered with other contexts, so different view controllers will have different perspectives on the data. Sometimes, though, it’s easier or more appropriate to retrieve the context from somewhere other than application or the document, or the view controller. Several objects you might use in a Core Data-based application keep a reference to a managed object context. A managed object itself has a reference to its own context, as do the various controller objects that support Core Data such as array and object controllers (NSArrayController and NSObjectController in OS X, and NSFetchedResultsController in iOS). Retrieving the context from one of these objects has the advantage that if you re-architect your application, for example to make use of multiple contexts, your code is likely to remain valid. For example, if you have a managed object, and you want to create a new managed object that will be related to it, you can ask original object for its managed object context and create the new object using that. This will ensure that the new object you create is in the same context as the original. Creating a New Managed Object Context You sometimes need to create a new managed object context to contain a disjoint set of edits that you might want to discard without perturbing the main context (for example, if you’re presenting a modal view to add and edit a new object). To create a new managed object context, you need a persistent store coordinator. Accessing the Core Data Stack Creating a New Managed Object Context 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 5 NSPersistentStoreCoordinator *psc = <#Get the coordinator#>; NSManagedObjectContext *newContext = [[NSManagedObjectContext alloc] init]; [newContext setPersistentStoreCoordinator:psc]; If you already have a reference to an existing context, you can ask it for its persistent store coordinator. This way you can be sure that the new context is using the same coordinator as the existing one (assuming this is your intent): NSManagedObjectContext *context = <#Get the context#>; NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator]; NSManagedObjectContext *newContext = [[NSManagedObjectContext alloc] init]; [newContext setPersistentStoreCoordinator:psc]; Getting the Managed Object Model and Entities You sometimes need to access a managed object model to get information about a particular entity. Applications typically have just a single model (although it may have more than one configuration). In a single-coordinator application, you typically get the model directly from the application delegate. In a document-based application, you get the model directly from the document. If you have access to a managed object context—directly or indirectly (see “Getting a Managed Object Context” (page 4))—you can get the model from the context’s persistent store coordinator. From the model, you can retrieve an entity using entitiesByName: NSManagedObjectContext *context = <#Get the context#>; NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator]; NSManagedObjectModel *model = [psc managedObjectModel]; NSEntityDescription *entity = [[model entitiesByName] objectForKey:@"<#Entity name#>"]; Accessing the Core Data Stack Getting the Managed Object Model and Entities 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 6 Creating a managed object: When you create a new managed object, you need to specify its entity. Typically, however, you don’t actually need to access the entity or model directly—see “Creating and Deleting Managed Objects” (page 16). Adding a Persistent Store In many applications, there is only one persistent store for each persistent store coordinator. In an single-coordinator application, the store is associated with the whole application. In a document-based application, each document has a separate store. Sometimes, however, you might want to add another store. You add the store to the persistent store coordinator. You have to specify the store’s type, location, and configuration (based on configurations present on the managed object model associated with the coordinator). You can also specify other options, such as whether an old version of the store should be migrated to a current version (see Core Data Model Versioning and Data Migration Programming Guide ). In an single-coordinator applications, you can get the application’s coordinator directly from the application delegate. In document-based applications, you can get the coordinator from the document’s managed object context. 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. } Accessing the Core Data Stack Adding a Persistent Store 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 7 This article contains snippets for fetching managed objects. To fetch managed objects, you minimally need a managed object context against which to execute the fetch, and an entity description to specify the entity you want. You create an instance of NSFetchRequest and specify its entity. You may optionally specify an array of sort orderings and/or a predicate. How you get the managed object context depends on your application architecture—see “Getting a Managed Object Context” (page 4). Once you have the context, though, you can get the entity using NSEntityDescription’s convenience method, entityForName:inManagedObjectContext:. Basic Fetch To get all the managed objects of a given entity, create a fetch request and specify just the entity: 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. } Fetch with Sorting To fetch managed objects in a particular order, in addition to the components in the basic fetch (described in “Basic Fetch” (page 8)) you need to specify an array of sort orderings: 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 8 Fetching Managed Objects 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. } Fetch with a Predicate To fetch managed objects that meet given criteria, in addition to the components in the basic fetch (described in “Basic Fetch” (page 8)) you need to specify 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]; Fetching Managed Objects Fetch with a Predicate 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 9 NSError *error; NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error]; if (fetchedObjects == nil) { // Handle the error. } For more about predicates, see Predicate Programming Guide . For an alternative technique for creating the predicate that may be more efficient, see “Fetch with a Predicate Template” (page 10). Fetch with a Predicate Template To fetch managed objects that meet given criteria, in addition to the components in the basic fetch (described in “Basic Fetch” (page 8)) you need to specify a predicate.NSPredicate’s predicateWithFormat: method is typically the easiest way to use a predicate (as shown in “Fetch with a Predicate” (page 9)), but it’s not the most efficient way to create the predicate itself. The predicate format string has to be parsed, arguments substituted, and so on. For performance-critical code, particularly if a given predicate is used repeatedly, you should consider other ways to create the predicate. For a predicate that you might use frequently, the easiest first step is to create a predicate template. You might create an accessor method that creates the predicate template lazily on demand: // Assume an instance variable: // NSPredicate *predicateTemplate; - (NSPredicate *)predicateTemplate { if (predicateTemplate == nil) { predicateTemplate = [NSPredicate predicateWithFormat:@"<#Key#> <#Operator#> <#$Variable#>"]; } return predicateTemplate; } When you need to use the template, you create a dictionary containing the substitution variables and generate the predicate using predicateWithSubstitutionVariables:. NSManagedObjectContext *context = <#Get the context#>; Fetching Managed Objects Fetch with a Predicate Template 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 10 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. } For more about predicates, see Predicate Programming Guide . Fetch with Sorting and a Predicate To fetch managed objects that meet given criteria and in a particular order, in addition to the components in the basic fetch (described in “Basic Fetch” (page 8)) you need to specify a predicate and an array of sort orderings. 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]; Fetching Managed Objects Fetch with Sorting and a Predicate 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 11 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. } For more about predicates, see Predicate Programming Guide . For an alternative technique for creating the predicate that may be more efficient, see “Fetch with a Predicate Template” (page 10). Fetching Managed Objects Fetch with Sorting and a Predicate 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 12 This article contains snippets for fetching specific attribute values for a given entity. Sometimes you don’t want to fetch actual managed objects; instead, you just want to retrieve—for example—the largest or smallest value of a particular attribute, or distinct values for a given attribute. On iOS, you can use NSExpressionDescription objects to specify a function for a fetch request, and setReturnsDistinctResults: to return unique values. To perform the fetch, you minimally need a managed object context against which to execute the fetch, and an entity description to specify the entity you want. How you get the managed object context depends on your application architecture—see “Getting a Managed Object Context” (page 4). Once you have the context, you can get the entity using NSEntityDescription’s convenience method, entityForName:inManagedObjectContext:. Fetching Distinct Values To fetch the unique values of a particular attribute across all instances of a given entity, you configure a fetch request with the method setReturnsDistinctResults: (and pass YES as the parameter). You also specify that the fetch should return dictionaries rather than managed objects, and the name of the property you want to fetch. 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. 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 13 Fetching Specific Property Values NSError *error; id requestedValue = nil; NSArray *objects = [context executeFetchRequest:request error:&error]; if (objects == nil) { // Handle the error. } Fetching Attribute Values that Satisfy a Given Function To fetch values that satisfy a particular function (such as the maximum or minimum value), you use an instance of NSExpressionDescription to specify the property or properties you want to retrieve. 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. Fetching Specific Property Values Fetching Attribute Values that Satisfy a Given Function 2009-07-06 | Copyright © 2009 Apple Inc. All Rights Reserved. 14 [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 setPropertiesToF