Core Data Relationships – Hacker Noon
While working on my latest project I have decided to write a tutorial about Core Data Relationships between Entities. Persistent Storage has. Studies of ecological relationships including food habits, reproduction, habitat TABLE 2. COMPARISON OF NlLGAl AND STEER CUTTING DATA. Nilgai. Steer. Dark green area represents the core zone, light green and yellow area . The data suggest that the nilgai exhibits substantial behavioural plasticity in .. the relationship between herd size and vigilance behaviour in data.
One-to-Many and Many-to-Many Core Data Relationships
Let me illustrate this with an example. We have a category that contains several notes. If the category is deleted, the notes are not notified of this event. The notes on the other end of the relationship believe that they are still associated with the deleted category. I have never had a need to use this delete rule in a project.
Adding Core Data entity relationships: lightweight vs heavyweight migration
In most situations, you want to take some action when a record is deleted. And that is where the other delete rules come into play. Nullify Delete Rule If the delete rule of a relationship is set to Nullify, the destination of the relationship is nullified when the record is deleted.
For example, if a category has several notes and the category is deleted, the relationships pointing from the notes to the category are nullified. This is the default delete rule and the delete rule you will find yourself using most often. Cascade Delete Rule The Cascade delete rule is useful if the data model includes one or more dependencies. Let me give you an example. If a note should always have a category, the deletion of a category should automatically delete the notes associated with that category.
In other words, the deletion of the category cascades or trickles down to the notes linked to the category. Even though this may make sense on paper, the user probably won't like it when you automatically delete its notes. Deny If there is at least one object at the relationship destination employeesdo not delete the source object department.
- Core Data Relationships
- One-to-Many and Many-to-Many Core Data Relationships
- Core Data Relationships and Delete Rules
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.
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.
Core Data Programming Guide: Creating Managed Object Relationships
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.
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.
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.