objective c - Fetched Properties v Relationships (Core Data - iPhone) - Stack Overflow
Adding Core Data entity relationships: lightweight vs heavyweight migration To do that, long press on the Add Entity button and choose Add Fetch Index from the menu using relationships, which are a bit like calculated properties except Core Data . Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design. Feb 26, Core Data. If you need to create a relationship between entities in different stores, you typically use fetched properties. Q: Is CoreData equals to SQLite or some wrapper? No, Core Data is Benjy -> Sales Gillian -> Sales. In Core Data, every relationship can have an inverse relationship. relationship, you might choose to use either a to-one or a to-many relationship for the inverse. Fetched. Properties. Relationships allow you to associate managed objects with Later, when you buy new songs from that artist, they are added to that Smart.
Before you release a Core Data app to the App Store, you can ignore versioning and edit the model as many times as you like. This will deploy a new persistent store using the updated model, and the crash will be resolved. From that point on you must add a new version if you update the model. Instead of releasing model versions 2, 3 and 4, use this trick to reduce version history: Share the love and tweet it to your friends! When you work with managed objects, you do so in memory using a managed object context.
Use reset to remove all managed objects from a context Use refreshObject: To increase your visibility of the number of objects in a context, log the results of [[context registeredObjects] count] to the debug console regularly. If large photos are within managed objects cut from the same entity that drives a table-view, performance will suffer. To get around this issue, attributes that will hold large objects should be split off into a related entity. This way the large objects can remain in the persistent store and can be represented by a fault instead, until they really are needed.
If you need to display photos in a table view, you should use auto-generated thumbnail images instead. Not Preloading Data When you ship a new model version with an updated application, you need to be careful not to accidentally ship a default data store based on an old model. If you do, the update will probably crash on launch for some if not all users. This threat can deter developers from shipping a default data store at all!
If default data is included with an application, it becomes easier to learn to use. The easier a program is to use, the more likely it is that people will continue to use it. The longer people use an application, the more chance that word about the application will spread, thus increasing sales potential.
To avoid crashing updates while still providing default data, you need a good testing strategy.
Adding Core Data entity relationships: lightweight vs heavyweight migration
You should deploy an untouched App Store version of the application to your device, add some data, and then test the upgrade process thoroughly. Not Using Multiple Contexts At a minimum, a Core Data implementation should have one context operating on the main thread. The user interface also operates on the main thread, so any slowdowns to the main thread will impact application responsiveness.
While it is easier to use only one context, performance issues can creep in unless your data set is extremely small. Cascade Delete the objects at the destination of the relationship when you delete the source. For example, if you delete a department, fire all the employees in that department at the same time. No Action Do nothing to the object at the destination of the relationship.
For example, if you delete a department, leave all the employees as they are, even if they still believe they belong to that department. It should be clear that the first three of these rules are useful in different circumstances.Learn about Relationships with Core Data - Beginning Core Data Tutorial - victoryawards.us
For any given relationship, it is up to you to choose which is most appropriate, depending on the business logic. It is less obvious why the No Action rule might be of use, because if you use it, it is possible to leave the object graph in an inconsistent state employees having a relationship to a deleted department.
Getting Started with Core Data Tutorial
If you use the No Action rule, it is up to you to ensure that the consistency of the object graph is maintained. You are responsible for setting any inverse relationship to a meaningful value. This may be of benefit in a situation where you have a to-many relationship and there may be a large number of objects at the destination.
Manipulating Relationships and Object Graph Integrity When you modify an object graph, it is important to maintain referential integrity. Core Data makes it easy for you to alter relationships between managed objects without causing referential integrity errors.
You may be wondering what an NSEntityDescription is all about. Recall earlier, NSManagedObject was called a shape-shifter class because it can represent any entity.
An entity description is the piece linking the entity definition from your Data Model with an instance of NSManagedObject at runtime. With an NSManagedObject in hand, you set the name attribute using key-value coding. You must spell the KVC key name in this case exactly as it appears in your Data Model, otherwise, your app will crash at runtime.
You commit your changes to person and save to disk by calling save on the managed object context. Note save can throw an error, which is why you call it using the try keyword within a do-catch block. Finally, insert the new managed object into the people array so it shows up when the table view reloads.
Some of the code here, such as getting the managed object context and entity, could be done just once in your own init or viewDidLoad then reused later. Build and run the app, and add a few names to the table view: If the names are actually stored in Core Data, the HitList app should pass the persistence test. With the app in the foreground, go to the fast app switcher and then terminate it. From Springboard, tap the HitList app to trigger a fresh launch. The table view is empty: You saved to Core Data, but after a fresh app launch, the people array is empty!
Fetching from Core Data To get data from your persistent store into the managed object context, you have to fetch it.
Before you can do anything with Core Data, you need a managed object context. Fetching is no different! Like before, you pull up the application delegate and grab a reference to its persistent container to get your hands on its NSManagedObjectContext.
Fetched Properties; useful? | Cocoa Is My Girlfriend
Fetch requests are both powerful and flexible. You can use fetch requests to fetch a set of objects meeting the provided criteria i. Fetch requests have several qualifiers used to refine the set of results returned.
For now, you should know NSEntityDescription is one of these required qualifiers. This is what you do here to fetch all Person entities. Also note NSFetchRequest is a generic type. You hand the fetch request over to the managed object context to do the heavy lifting. If an error occurred during the fetch, you can inspect the error inside the catch block and respond appropriately.
Build and run the application.
Immediately, you should see the list of names you added earlier: Add a few more names to the list and restart the app to verify saving and fetching are working. Short of deleting the app, resetting the Simulator or throwing your phone off a tall building, the names will appear in the table view no matter what. There were a few rough edges in this sample app: Key Points Core Data provides on-disk persistence, which means your data will be accessible even after terminating your app or shutting down your device.
This is different from in-memory persistence, which will only save your data as long as your app is in memory, either in the foreground or in the background. Xcode comes with a powerful Data Model editor, which you can use to create your managed object model. A managed object model is made up of entities, attributes and relationships An entity is a class definition in Core Data.
An attribute is a piece of information attached to an entity. You can read and write to its attributes using Key-Value Coding. Where to Go From Here?
Core Data Programming Guide: Creating Managed Object Relationships
Data Models, entities, attributes, managed objects, managed object contexts and fetch requests. If you enjoyed what you learned in this tutorial, why not check out the complete Core Data by Tutorials bookavailable in our store? This chapter covers the basics of setting up your data model and then adding and fetching records.
This chapter will teach you how you customize your own managed object subclasses to store and validate data. Chapter 3, The Core Data Stack: Under the hood, Core Data is made up of many parts working together. Chapter 4, Intermediate Fetching: