How to group values by the Core Data attribute
The executeFetchRequest: It either returns an NSArray object with zero or more objects, or it returns nil. If nil is returned, you have received an error from Core Data and need to respond to it. If the array exists, you receive possible results for the request even though the NSArray may be empty.
An empty NSArray indicates that there were no records found.
The real flexibility in fetching objects comes in the complexity of the fetch request. To begin with, you can add an NSPredicate object to the fetch request to narrow the number of objects being returned.
Cleaning up Core Data Fetch Requests
In addition to narrowing the objects being returned, you can configure how those objects are returned. Further, you can configure the NSFetchRequest so that those NSDictionary instances only contain a subset of the properties available on the Employee entity. For more information about NSFetchRequest , see the class documentation. We only want to seed it once. We then loop through an array of strings, the names for the list records, and create a list record for each string. We set the name and createdAt attributes of the list record and add ten item records to each list record.
Note that we mark some of the item records as completed by setting their completed attribute to true.
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- Core Data Queries Using Expressions?
- Core Data Programming Guide: Fetching Objects;
- Examples of using NSPredicate to filter NSFetchRequest.
We also set the list relationship of each item record to the current list record. We push the changes to the persistent store by saving the managed object context and we set didSeedPersistentStore to true in the user defaults database.
If your application takes too long to launch, the operating system's watchdog kills your application. The watchdog has no mercy. The order of the records returned by a fetch request is undefined. The users of your application expect more from you, though.
Getting Started: Editing, Not Just for Writers
Sorting records is a common task. We create a fetch request for the List entity.
To sort the records returned by the fetch request, we add a sort descriptor, an instance of the NSSortDescriptor class. We initialize it by invoking its init key: We then set the sortDescriptors property of the fetch request. The sortDescriptors property is of type [NSSortDescriptor] , which means we can combine multiple sort descriptors. The order of the sort descriptors determines in which order the sort descriptors are applied.
- Core Data on iOS 5 Tutorial: How To Work with Relations and Predicates.
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- More Fetching and Deleting Managed Objects With Core Data;
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We can use predicates to refine the result of a fetch request. Predicates, instances of the NSPredicate class, enable us to specify which records we are interested in. Update the previous example as shown below. The syntax of the predicate is reminiscent of SQL.
Examples of using NSPredicate to filter NSFetchRequest - a free Hacking with Swift tutorial
The predicate defines that we are only interested in list records with a name that includes or contains the letter o. The [c] specifier indicates that the case of the letter o should be ignored. This is what the result looks like in the console. This example fetches item records and sorts them by name.
In the example, we instantiate two predicates and turn them into a compound predicate , using the NSCompoundPredicate class.
The first predicate restricts the fetch request to item records that are marked as completed. The second predicate specifies that we are only interested in item records that belong to a list record that has a name equal to Home. Run the application and inspect the result in the console.
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