Core Data on iOS 5 Tutorial: How To Work with Relations and Predicates | animesost.info
Explains how to manage objects using the Core Data framework. A relationship fault is a subclass of the collection class that represents the. Faulting is a concept that often confuses developer new to Core Data. of the entities, attributes, and relationships defined in the data model. I've been using Core Data since iOS I was working Define entities with properties (called attributes) and relationships with other entities (one-to-one, one-to-many, ConcurrencyDebug” to iOS 8, making it easier for us to test against concurrency issues. fault, faultingState, hasChanges, and entity.
Add it to the end of the file: Add the necessary code again to the end of the file: Add the following code below viewDidLoad: Add the code for those to the end of the file: Before showing the date picker, the first responder for all text fields is resigned, thus effectively dismissing the keyboard if it was visible.
To test your new view, you need to push it onto the navigation stack when a cell is tapped. Run the app and create a few instances of banks. Each bank record is editable, including the close date. To save data, hit the save button; to discard changes, just tap the back button. Notice that the date picker and the keyboard never obstruct each other. Changes are reflected in the list of banks with no need to refresh the table view. Think of the classic example of employees and departments — an employee is said to belong to a department, and a department has employees.
In database modeling, relationships can be of three types: This property is usually referred to as cardinality. In the example from the previous section, there is already a relation modeled in Core Data: This is a one-to-one relationship: The graphical view stresses this point by connecting the two entities with one single arrow line.
Core Data on iOS 5 Tutorial: How To Work with Relations and Predicates
In other words, these two only have eyes for each other. This is just a string identifying the name of the relation. This is the target or the destination class of the relation. The answer to the question: Is the destination a single object or not? If yes, the relation is of type to-one, otherwise it is a to-many. The definition of the inverse function. It is pretty rare to find a domain where this is not needed.
It is also a sort of logical necessity: In your example, a department can have more than one employee, so this is a to-many relation.
As a general rule, a one-to-many relation has a many-to-one inverse. In case you want to define a many-to-many relationship, you simply define one relation as to-many and its inverse as a to-many as well. Make sure you define an inverse for each relationship, since Core Data exploits this information to check the consistency of the object graph whenever a change is made.
This defines the behavior of the application when the source object of a relationship is deleted. For the delete rule in 5 above, there are four possible values: Nullify is the simplest option. They just keep thinking they have not been fired: If you select cascade as the delete rule, then when you delete the source object it also deletes the destination object s. Such a rule is appropriate only if you want to close a department and fire all of its employees as well.
In this case it is enough to set the delete rule for department to cascade and delete that department record. Deny, on the other hand, prevents accidental deletions.
Delete rules have to be specified for both sides of a relationship, from employee to department and vice versa. Each domain implements its own business logic, so there is no general recipe for setting delete rules. Just remember to pay attention when using the cascade rule, since it could result in unexpected consequences. To maximize the performance of your application, remember this when you devise your data model and try to use relationships only if necessary.
The first step is to add a new entity. The delete rule is the default, nullify 4. As above, this is a to-many relationship 3 with a delete rule of nullify. A new class, named Tag, will pop up in your project tree. Sometimes, quite often, in fact: If this happens to you, select one set of instances and delete them, but choose to remove references rather than to trash the files. At this point, you have changed the Core Data model, so your app will not be compatible with the old model on your device.
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.
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.02 Core Data Relationships - Define Entities and Relationships - Expenses with Categories App
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. A fault is a placeholder object that represents a managed object that has not yet been fully realized or a collection object that represents a relationship: A managed object fault is an instance of the appropriate class, but its persistent variables are not yet initialized.
What Is a Core Data Fault
A relationship fault is a subclass of the collection class that represents the relationship. Faulting allows Core Data to put boundaries on the object graph. Because a fault is not realized, a managed object fault consumes less memory, and managed objects related to a fault are not required to be represented in memory at all.
To illustrate, consider an application that allows a user to fetch and edit details about a single employee. The employee has a relationship to a manager and to a department, and these objects in turn have other relationships. If you retrieve just a single Employee object from a persistent store, its manager, department, and reports relationships are initially represented by faults.
Figure A department represented by a fault Although the fault is an instance of the Department class, it has not yet been realized—none of its persistent instance variables have yet been set.
If it were a requirement that the object graph be complete, then to edit a single attribute of a single employee, it would ultimately be necessary to create objects to represent the whole corporate structure. Firing Faults Fault handling is transparent—you do not have to execute a fetch to realize a fault.
If at some stage a persistent property of a fault object is accessed, Core Data automatically retrieves the data for the object and initializes the object. This process is commonly referred to as firing the fault. If you access a property on the Department object — its name, for example — the fault fires and Core Data executes a fetch for you to retrieve all of the object's attributes. See NSManagedObject for a list of methods that do not cause faults to fire. Core Data automatically fires faults when a persistent property such as firstName of a fault is accessed.
However, firing faults individually can be inefficient, and there are better strategies for getting data from the persistent store see Decreasing Fault Overhead. When a fault is fired, Core Data does not go back to the store if the data is available in its cache.
With a cache hit, converting a fault into a realized managed object is very fast—it is basically the same as normal instantiation of a managed object.