Ios core data relationship fault on managed

It's not your fault

ios core data relationship fault on managed

Faults are an essential component of Core Data. Core Data is very good at what it does thanks to the hard work of Apple's Core Data team. Core . 2: Relationship 'items' fault on managed object (0xe5d0b0)

It's not your fault

A note can be associated with a category and a category can contain multiple notes. In other words, the Note and Category entities share a one-to-many relationship.

A note can also have tags and a tag can be linked to multiple notes. The Note and Tag entities share a many-to-many relationship. Build and run the application to give it a try. Make sure Notes contains a few notes, a few categories, and a few tags.

That is important for our experiment. Execute the fetch request and print the first note to the console. Run the application to see the result. We fetched a record of the Note entity. But notice that the data associated with the record is missing. Instead we see the word fault. As you know, Core Data can only operate on records of the persistent store once they are loaded into memory. This is only possible because Core Data is heavily optimized to keep its memory footprint as low as possible.

One of the techniques Core Data uses to accomplish this is faulting. But the team at Apple didn't invent faulting. Several other frameworks use a similar strategy to accomplish similar goals.

Ruby on Rails and Ember come to mind. Even though faulting may look mysterious at first, the idea is simple. Core Data only fetches the data it absolutely needs to satisfy the needs of your application. That is faulting in a nutshell. The idea of faulting is simple, but the underlying implementation is an advanced bit of programming. Fortunately, we don't have to worry about that. That is the responsibility of the framework.

Let me show you how it works with another example. Below the print statement, we safely unwrap the value of the title property and print it to the console and we add another print statement for the note.

ios core data relationship fault on managed

We print the note to the console, ask the value of one of the properties of the note, and print the note again. Why we do that becomes clear when we inspect the results in the console. Run the application and take a look at the output in the console. Despite this fault, we can access the value of the title property and print it to the console. This is confirmed by the third print statement in which we print the note again. What is happening here?

We first asked for the user's notes and Core Data diligently gave us the list of notes. But, as you can see in the console, it is a list of empty records. From the moment we ask for the value of a property of one of the records, Core Data jumps into action and fetches the data from the persistent store.

This is better known as firing a Core Data fault. But it doesn't just fetch the value of the title property. As you can see in the console, Core Data fetches the values of every property of the note with the exception of relationships.

Notice that the value of the tags property is missing. Instead, Xcode displays relationship fault. This means that the tags of the note haven't been fetched yet. If the data is not available in the cache, Core Data automatically executes a fetch for the fault object; this results in a round trip to the persistent store to fetch the data, and again the data is cached in memory. Whether or not an object is a fault simply means whether or not a given managed object has all its persistent attributes populated and is ready to use.

If you need to determine whether an object is a fault, call its isFault method without firing the fault without accessing any relationships or attributes. If isFault returns NOfalse, then the data must be in memory and therefore the object is not a fault. However, if isFault returns YEStrue, it does not imply that the data is not in memory. The data may be in memory, or it may not, depending on many factors influencing caching. You are strongly discouraged from overriding description in this way.

There is no way to load individual attributes of a managed object on an as-needed basis and avoid realizing retrieving all property values of the entire object. Turning Objects into Faults Turning a realized object into a fault can be useful in pruning the object graph, as well as ensuring that property values are current. Turning a managed object into a fault releases unnecessary memory and sets its in-memory property values to nil. You can turn a realized object into a fault with the refreshObject: If there are, and you then save the context, you will introduce referential integrity problems to the persistent store.

What Is a Core Data Fault

When an object turns into a fault, its didTurnIntoFault method is called. Note Core Data avoids the term unfaulting because it is confusing. Page faults are triggered, caused, fired, or encountered. Core Data describes this as "turning an object into a fault. If you are observing properties of an object that is turned into a fault and the fault is subsequently realized, you receive change notifications for properties whose values have not in fact changed.

Although the values are not changing semantically from your perspective, the literal bytes in memory are changing as the object is materialized. The key-value observing mechanism requires Core Data to issue the notification whenever the values change according to the perspective of pointer comparison.

KVO needs these notifications to track changes across key paths and dependent objects. Uniquing Ensures a Single Managed Object per Record per Context Core Data ensures that—in a given managed object context—an entry in a persistent store is associated with only one managed object. The technique is known as uniquing. Without uniquing, you might end up with a context maintaining more than one object to represent a given record.

For example, consider the situation illustrated in Figure ; two employees have been fetched into a single managed object context. Each has a relationship to a department, but the department is currently represented by a fault. Figure Independent faults for a department object It would appear that each employee has a separate department, and if you called department on each employee—turning the Department faults into regular objects—you would have two separate Department objects in memory.

However, if both employees belong to the same department for example, MarketingCore Data ensures that in a given managed object context only one object representing the Marketing department is ever created.

ios core data relationship fault on managed

If both employees belong to the same department, their department relationships would both therefore reference the same fault, as illustrated in Figure Figure Uniqued fault for two employees working in the same department Without uniquing, if you fetched all the employees and called department on each—thereby firing the corresponding faults—a new Department object would be created every time.