One-to-Many and Many-to-Many Core Data Relationships
I was working on my first application: a weather app providing past weather data as I did not really need to use Core Data, but I wanted to gain some Define entities with properties (called attributes) and relationships with. many-to-many relationships in Core Data, especially when working with Swift. The Account entity has no attributes. As of Xcode , Xcode automatically generates a class definition for every entity of the data model. It is not. Core Data is an “object graph and persistence framework”, which of data with a defined organization) of objects and then persist them in some fashion. make sure bidirectional relationships are properly hooked up.
You can have only one to-many element in a key path in a predicate. For example, no toOne. CoreData supports a noindex: This is done primarily for performance reasons: SQLite uses a limited number of indices per query, and noindex: See NSPredicate documentation regarding function expressions.
However, the SQLite store does not generally support writing directly to file systems that do not implement byte-range locking.
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Similarly, the file size is reduced if you remove an item that itself occupies multiple pages such as a thumbnail image. An SQLite file is organized as a collection of pages.
The data within those pages is managed through B-trees, not as simple fixed-length records. This format is more efficient for searching and for overall storage, because it allows SQLite to optimize how it stores both data and indexes in a single file.
However, the cost of this design is that some delete operations may leave holes in the file and impact read and write performance. Loss of that partial update would be catastrophic, so ensure that the file is written correctly before your application continues.
Unfortunately, doing partial file updates means that in some situations saving even a small set of changes to an SQLite store can take considerably longer than saving to, say, an XML store.
For example, where saving to an XML file might take less than a hundredth of a second, saving to a SQLite store may take almost half a second. This data loss risk is not an issue for XML or binary stores.
Because writes to these stores are typically atomic, it is less likely that data loss involves corruption of the file, and the old file is not deleted until the new one has been successfully written. This request causes the kernel to flush all buffers to the drives and causes the drives to flush their track caches.
Without this, there is a significantly large window of time within which data will reside in volatile memory.
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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.
One-to-Many and Many-to-Many Core Data Relationships
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. 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.
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.Swift 4: Using Core Data - Entities, Attributes and Relationships - Lesson 2
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: You define a custom subclass of NSManagedObject for a particular entity and implement custom accessors methods and perhaps dependent keys.
At runtime, the accessor methods are not called, and the dependent key is not updated. In the model, you did not specify the custom class for the entity. Ensure that the model specifies the custom class name for the entity rather than NSManagedObject.
You create a sort descriptor that uses a comparison method defined by NSStringsuch as the following: You then either use this descriptor with a fetch request or as one of an array controller's sort descriptors. At runtime, you see an error message that looks similar to the following: Exactly how a fetch request is executed depends on the store—see Fetching Objects. If you are executing the fetch directly, do not use Cocoa-based sort operators—instead, sort the returned array in memory.
If you are using an array controller, you may need to subclass NSArrayControllerso that it will not pass the sort descriptors to the database but will instead do the sorting after your data has been fetched. Problems with Saving Cannot save documents because entity is null Problem: You have a Core Data document-based application that is unable to save. When you try to save the document you get an exception: Exception raised during posting of notification.
Cannot perform operation since entity with name 'Wxyz' cannot be found Cause: In short, you have a controller in entity mode with an invalid entity name. Select in turn each of your controllers in Interface Builder, and press Command-1 to show the inspector. For each controller, ensure you have a valid entity name in the Entity name field at the top of the inspector.
You add an object to a context. When you try to save the document you get an error that looks like this: Basically, it's checking to see if any other code thread, process, or just a different managed object context changed this object without your knowledge.
Core Data does not do this check on newly inserted objects because they could not have existed in any other scope. They haven't been written to the database yet. You may have forced a newly inserted object to lose its inserted status and then changed or deleted it.
You may have passed an inserted object to another managed object context. There are a number of possible remedies, depending on the root cause: Do not pass an inserted not yet saved object to another context.
Only objects that have been saved can be passed between contexts.