Importing items from network to a Core Data store

Many apps use Core Data for persistence and also need to import data from a server. Imported items typically have a unique identifier which can be used for identifying them. The count of imported items can be high therefore it is preferred to batch insert the items instead of adding them one by one. Core Data framework has a specialized request for this called NSBatchInsertRequest which is available since iOS 13. If we combine batch insert with Core Data constraints then we can achieve a flow where new items are only created when the store does not have an item for the unique identifier. All the other items already available in the persistent store are updated (instead of deleting the old item and reinserting it). In this blog post let’s take a look on how it works with a sample app which displays a list of Product entities with a name and a unique serial code attributes.

Constraints on the entity can be set in the model editor. For making sure that only one Product with a serial code of X exists in the persistent store then we will need to add a constraint on the serialCode attribute. Core Data framework will then make sure that only one entity with unique serial code exists in the persistent store. Neat, no need to query the store first for existing products and manually checking for possible duplicates.

With a constraint set up, let’s take a look on the batch insert. Apple added NSBatchInsertRequest to Core Data framework in iOS 13. As we added a constraint then we need to tell Core Data what to do if there is already an item for the unique serial code. If we set NSManagedObjectContext‘s merge policy to NSMergeByPropertyObjectTrumpMergePolicy before executing the batch insert then Core Data goes and updates existing items with incoming attribute values fetched from a server. If there is not an item in the store with serial code then a new item is inserted. In summary, we get a behaviour where existing items are updated and missing items are inserted when importing items from a server. The flow of fetching data from a server, running batch insert on a background context and then refreshing fetched results controller can be seen below.

	final class ViewModel: ObservableObject {
	func importProducts() {
	ProductAPI.getAll { result in
	switch result {
	case .success(let products):
	self.persistenceController.container.performBackgroundTask { context in
	context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
	let batchInsert = NSBatchInsertRequest(entityName: "Product", objects: products)
	do {
	let result = try context.execute(batchInsert) as! NSBatchInsertResult
	print(result)
	}
	catch {
	let nsError = error as NSError
	// TODO: handle errors
	}
	DispatchQueue.main.async {
	objectWillChange.send()
	// TODO: handle errors
	try? resultsController.performFetch()
	}
	}
	}
	}
	}
	}

view raw ViewModel.swift hosted with ❤ by GitHub

Summary

NSBatchInsertRequest is a welcoming change which makes it easy to insert and update existing items already in the persistent store. Setting up a constraint on a unique identifier and setting merge policy on a context enables us to handle SQL upserts without much code.

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Example Project

CoreDataBatchInsertConstraints (Xcode 12.3)