Ios – Core data: any way to fetch multiple entities

core-dataiosiphone

I'm just getting started with Core Data, and as a learning exercise I'm building an app where I need to display different types of objects in a single table view.

As an example, say I have an entity for "Cheese" and an unrelated entity for "Pirate". On the main screen of my app, the user should be able to create either a "Cheese" or a "Pirate" instance to add to the table view.

So, using the core data editor I've created entities for Cheese and Pirate… however, an NSFetchRequest seems to only allow you to retrieve one type of entity at a time with:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Cheese" inManagedObjectContext:_context];
[fetchRequest setEntity:entity];

Is there any way to perform a fetch that retrieves all "Cheese" and "Pirate" objects?

Thanks.

Best Answer

What you're trying to do is accomplished by defining entity inheritance in your model, where "DisplayableObject" would be an abstract class defined as the parent of "Cheese" and "Pirate".

Then you can perform a fetch request on the "DisplayableObject" entity and it will retrieve both entities' objects.

Take a look into this article in the apple documentation: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CoreData/KeyConcepts.html

iOS 9 and later:

iOS 9 added a new class called NSBatchDeleteRequest that allows you to easily delete objects matching a predicate without having to load them all in to memory. Here's how you'd use it:

Swift 5

let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Car")
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)

do {
    try myPersistentStoreCoordinator.execute(deleteRequest, with: myContext)
} catch let error as NSError {
    // TODO: handle the error
}

Objective-C

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Car"];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];

NSError *deleteError = nil;
[myPersistentStoreCoordinator executeRequest:delete withContext:myContext error:&deleteError];

More information about batch deletions can be found in the "What's New in Core Data" session from WWDC 2015 (starting at ~14:10).

iOS 8 and earlier:

Fetch 'em all and delete 'em all:

NSFetchRequest *allCars = [[NSFetchRequest alloc] init];
[allCars setEntity:[NSEntityDescription entityForName:@"Car" inManagedObjectContext:myContext]];
[allCars setIncludesPropertyValues:NO]; //only fetch the managedObjectID

NSError *error = nil;
NSArray *cars = [myContext executeFetchRequest:allCars error:&error];
[allCars release];
//error handling goes here
for (NSManagedObject *car in cars) {
  [myContext deleteObject:car];
}
NSError *saveError = nil;
[myContext save:&saveError];
//more error handling here

Nginx – HTTPConnectionPool(host=’0.0.0.0′, port=5000): Max retries exceeded with url

The error comes from the use of 0.0.0.0

Docker introduces a virtual network, so each docker-isntance sees itself as 127 not the host machine

(thx @Jmons)

To launch your code locally, use your fixed IPv4 address.

For Windows users: open a command prompt (cmd) and run the ipconfig command, retrieve the @IP which is atIPv4 address. . . . . . . . . . . . . .:it is generally of type 192.168.x.x IPv4 address in command prompt

Best Answer

Related Solutions

Ios – Core Data: Quickest way to delete all instances of an entity

iOS 9 and later:

Swift 5

Objective-C

iOS 8 and earlier:

Nginx – HTTPConnectionPool(host=’0.0.0.0′, port=5000): Max retries exceeded with url

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