Objective-C Fundamentals

236 CHAPTER 12 Reading and writing application data
12.4.3 Defining the relationships
As the Person and Task models stand right now, they won’t be of much use. You want
to be able to add multiple tasks to a person. That means you must create a relationship
between both of your entities: a task “belongs to” a person, and a person “has
many” tasks.
With the Data Modeling tool in the Table Editor style, select the Person entity and
click the + button in the Relationships section. Call the relationship “tasks,” and select
the Task entity as its destination. In the Data Model Inspector pane, next to Plural,
check the To-Many Relationship box to tell Core Data that the Person entity can have
more than one task. Finally, set the Delete Rule to Cascade. Cascade causes all the
tasks that belong to a given Person entity to be deleted when the Person entity is
deleted. Take, for example, a person called Peter. Peter has three tasks: go shopping,
clean the car, and learn Core Data. When you delete Peter, those three tasks are
deleted as well. The other two settings do the following:
■
■
■
Nullify—The child entities (in this case, the tasks) won’t be deleted, but they
won’t “belong” to the deleted Person entity anymore. They’ll be orphans.
Deny—Core Data won’t allow a parent entity to be deleted as long as it still has
children. In this case, Peter can’t be deleted if he still has three tasks.
No Action—Leave the destination objects as they are. That means they’ll most
likely point to a parent entity that no longer exists. In this case, the three tasks
would still belong to Peter even though he was deleted—a situation you generally
want to avoid.
You’ve specified that a person has many tasks; now you must specify that a task belongs
to a person. Select the Task entity and add a new relationship called “person.”
Uncheck Optional because you don’t want to be able to create unassigned tasks. The
destination entity is obviously Person. The Inverse Relationship field lets you select
the corresponding relationship in the destination entity (in this case, the tasks relationship
of the Person entity). Why is that important? For data integrity, you always
want to connect both sides of a relationship: you must always be able to access a person
object’s tasks, and you must always be able to access a task object’s person. That
way, you can change a task’s assignment from Peter to Mary, and Core Data knows it
must update the task’s person to Mary and update Peter’s and Mary’s lists of tasks.
With inverse relationships, you can always be sure your data stays consistent and Core
Data does The Right Thing. Never forget to set up the inverse relationship.
Leave the Delete Rule at Nullify because you don’t want a person to be deleted just
because one of their tasks is deleted, but don’t want a finished task to stay connected
to the person before it’s deleted.
Your tasks relationship should look like figure 12.2, and your person relationship
should look like figure 12.3.
You’re done with the data model. Make sure to save it before moving on to the
next step.