ios - Core Data attribute on relationship? - Stack Overflow
Poor performance, for example, may not be a Core Data problem, but instead it is next accessed, its data values are retrieved from its persistent store. If no inverse relationship exists and an object is deleted, you will be. Attributes declared as transient are never saved to the persistent store, but they The documentation still states that Core Data cannot automatically generate accessor you can define unidirectional relationships and Core Data will not complain. sometimes it is trivial to do, and other times it can create major headaches. It also touches on Core Data's benefits, terminology, versioning, migration, An object that mediates between Persistent Store(s) and Managed Object The ability to organize data into entities structured with relationships is where Core Data not only leverages the benefits of a relational database, you.
Migraine disorders, chronic migraine, episodic migraine, epidemiology, headache Background Migraine is a common and often debilitating condition, with a one-year prevalence of Migraine is generally divided into two groups based on headache days per month.
However, the definition of CM has evolved over the past 25 years 4most recently with the release of ICHD-3 beta 5. EM and CM have been well characterized in population studies from many regions of the world 26 — 14and results have been summarized in meta-analyses 15 These studies found significant headache-related disability associated with CM and EM 26 — 8111517although more so with CM 691117 — In comparison with people with EM, those with CM experience substantially greater headache impact on daily activities 1120higher direct medical costs 92122greater health care resource utilization 1011reduced health-related quality of life 1123and higher rates of comorbidities 911 Also, many studies have defined risk factors for migraine chronification, which have been summarized in multiple review articles 1724 — Despite previous in-depth population studies, much remains to be discovered about the effects of EM and CM on individuals.
This means that even minor changes require that the whole file be loaded into memory, and then the whole file be written back to disk when it is saved. As the data grows, the application will slow. This keeps memory usage low, which in turn ensures that the app remains responsive and prevents memory pressure crashes. Essentially, Core Data is more scalable than property lists because it supports using a database as a persistent store.
The ability to organize data into entities structured with relationships is where more of its true power lies. For example, consider the following entity that represents a task: Without relationships, this data model supports only one subtask.
Now consider the following entities: The line with the double-headed arrow indicates a to-many relationship from the Task entity to the Subtask entity. This means that a task can now have multiple subtasks, not to mention that reference to the parent task can be obtained through the inverse relationship!
This flexibility is not only more convenient, but also saves space in the database because the parent task name only has to be stored once. If you wanted to get really advanced and enable support for tasks to have subtasks of subtasks, what then? Consider the re-architected Task entity below: The model now supports an unlimited depth of subtasks because the Task entity is related to itself!
The scalability and flexibility of Core Data is only scratching the surface of its benefits. Core Data takes on that responsibility for you, and optimizes the generated SQL for you automatically. If the structure of the model ever changes, then the persistent store must be updated to match.
If your customers are using stores based on a model you have since edited without versioning, your app is destined to crash.
swift - Core Data not saving the relationship - Stack Overflow
As a side note, some changes such as attribute defaults, validation rules, and fetch request templates can be modified without consequence.
Using Model Versioning and Migration Too Much Once developers come to terms with how simple it is to maintain versions of a managed object model, some have a tendency to overuse it. This can lead to an overcomplicated version history if you add a version for every change, which only slows down the model migrations.
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. Either set a merge policy on the context, or manually programmatically resolve the failure.
You can retrieve the currently committed values for an object using committedValuesForKeys: Assigning a managed object to a different store Problem: You see an exception that looks similar to this example. The object you are trying to assign to a store has already been assigned and saved to a different store. To move an object from one store to another, you must create a new instance, copy the information from the old object, save it to the appropriate store, and then delete the old instance.
Fault cannot be fulfilled Problem: The object that Core Data is trying to realize has been deleted from the persistent store. Discard this object by removing all references to it. This problem can occur in at least two situations: You started with a strong reference to a managed object from another object in your application. You deleted the managed object through the managed object context. You saved changes on the object context.
At this point, the deleted object has been turned into a fault. Core Data will try to realize the faulted managed object but will fail to do so because the object has been deleted from the store.
That is, there is no longer an object with the same global ID in the store. You deleted an object from a managed object context. The deletion failed to break all relationships from other objects to the deleted object. At this point, if you try to fire the fault of a relationship from another object to the deleted object, it may fail, depending on the configuration of the relationship, which affects how the relationship is stored.
The delete rules for relationships affect relationships only from the source object to other objects including inverses.
Without potentially fetching large numbers of objects, possibly without reason, there is no way for Core Data to efficiently clean up the relationships to the object.
Avoiding Ten Big Mistakes iOS Developers Make with Core Data
Keep in mind that a Core Data object graph is directional. That is, a relationship has a source and a destination. Following a source to a destination does not necessarily mean that there is an inverse relationship. So, in that sense, you need to ensure that you are properly maintaining the object graph across deletes.
Core Data uses inverse relationships to maintain referential integrity within the data model.
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If no inverse relationship exists and an object is deleted, you will be required to clean up that relationship manually. In practice, a well-designed object graph does not require much manual post-deletion clean-up. Most object graphs have entry points that in effect act as a root node for navigating the graph, and most insertion and deletion events are rooted at those nodes just like fetches.
This means that delete rules take care of most of the work for you. Similarly, because smart groups and other loosely coupled relationships are generally best implemented with fetched properties, various ancillary collections of entry points into the object graph generally do not need to be maintained across deletes, because fetched relationships have no notion of permanence when it comes to objects found through the fetched relationship.
Managed object invalidated Problem: You see an exception that looks similar to this example: Either you have removed the store for the fault you are attempting to fire, or the managed object's context has been sent a reset. If you add the store again, you can try to fetch the object again. Class is not key-value coding-compliant Problem: You see an exception that looks similar to the following example.
Either you used an incorrect key, or you initialized your managed object with init instead of initWithEntity: Use a valid key check the spelling and case carefully—also review the rules for key-value coding compliance in Key-Value Coding Programming Guide.
Entity class does not respond to invocations of custom methods Problem: You define an entity that uses a custom subclass of NSManagedObject, then in code you create an instance of the entity and invoke a custom method, as illustrated in this code fragment: In the model, you misspelled the name of the custom class for the entity.
Ensure that the spelling of the custom class name in the model matches the spelling of the custom class you implement. Custom accessor methods are not invoked, and key dependencies are not obeyed Problem: