Core data to many relationship does not have an inverse – A Warning about your inverse relationships

core data to many relationship does not have an inverse

As Apple states, it is not a database, but instead a persistence you can add as many entities\tables your model needs to have. This is not strictly required, but setting the inverse relationship will help Core Data ensure the. The truth is, with Xcode 8 and Swift 3, it has never been easier to work with Core Data relationships. Let me show you how. The Account entity has no attributes. Don't forget to set the inverse relationship to users. Creating. You define a many-to-many relationship using two to-many relationships. You then set each to be the inverse of the other. don't worry: if you use an SQLite store, Core Data automatically For example, if an employee can have more than one manager (and a.

Designing a simple User Interface Since the main goal of this article is not related to how we design user interfaces, I will give you a quick setup to support this article.

To start, go ahead and select the Main.

core data to many relationship does not have an inverse

It should look like the picture below. Next, we need to indicate to the Table View which object will be responsible to return objects for it.

Core Data Relationships

Going back to code, add the following instance and method into the ViewController. And with this we have a basic user interface to support this use case for Code Data. I will not extend all user interfaces to fulfill the entire model layer, but you have the idea how to propagate to additional view controllers.

Then add to your ViewController. To fetch objects from the database the best aproach, and the solution Apple wants us to use, is the NSFetchedResultsController. Fetched results controllers can fetch efficiently records from a predicate and a sort order.

core data to many relationship does not have an inverse

It can also monitor changes to objects in the associated managed object context, and report changes in the result sets to its delegate. After creating it, we will use a var of the same type to access this fetched results controller instance. With this, we will only initialize the controller at the moment that var will be used, similiar to a lazy aproach.

The benefits of this are that we guarantee the instance will be initialized at the moment it will be used. And whenever a change occurs, the fetched results controller will call the method controllerDidChangeContent: Go ahead, include the method and call the reload data from the table view variable to refresh his data. First the Table View will ask how many sections our data contains.

One-to-Many and Many-to-Many Core Data Relationships

Then the number of rows for each section. And the updated cell for each object. At this point, if you run the application, you will only see the user interface that we have created before.

For editing and deleting, we will implement the optional delegate method the Table View has to offer. Include the following code into the ViewController. Many thanks for reading! The final project can be found on the following link: MyTasks - A simple Core Data examplegithub. Data persistence is always important for any application.

core data to many relationship does not have an inverse

There are so many tools we use for this purpose, but as an iOS Developer, this is one of my favourite, and I use it almost on every project. I work at Runtime Revolution where I build mobile products for our clients, using this framework and so many others. For example, if you want to remove a department, you must ensure that all the employees in that department are first transferred elsewhere; otherwise, the department cannot be deleted.

Nullify Remove the relationship between the objects, but do not delete either object. This only makes sense if the department relationship for an employee is optional, or if you ensure that you set a new department for each of the employees before the next save operation.

One-to-Many and Many-to-Many Core Data Relationships

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. 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.

core data to many relationship does not have an inverse

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. Much of this behavior derives from the relationship descriptions specified in the managed object model.

core data to many relationship does not have an inverse

When you need to change a relationship, Core Data takes care of the object graph consistency maintenance for you, so you need to change only one end of a relationship. This feature applies to to-one, to-many, and many-to-many relationships.

Consider the following examples.