In this tutorial, we take a look at the NSManagedObject class, a key class of the Core Data framework. You learn how to create a managed object, what classes are involved, and how a managed object is saved to a persistent store.
At first glance, NSManagedObject instances may appear to be glorified dictionaries. All they seem to do is manage a collection of key-value pairs. It is true that the NSManagedObject class is a generic class, but it implements the fundamental behavior required for model objects in Core Data. Let me explain what that means.
Every NSManagedObject instance has a number of properties that tell Core Data about the model object. The properties that interest us most are entity and managedObjectContext.
Every managed object has an entity description, an instance of the NSEntityDescription class. The entity description is accessible through the entity property.
Do you remember that we explored and defined entities in the tutorial about data models? An instance of the NSEntityDescription class represents an entity of the data model.
The entity description refers to a specific entity of the data model and it knows about the attributes and relationships of that entity. Every managed object is associated with an entity description. No exceptions.
A managed object should always be associated with a managed object context. There are no exceptions to this rule. Remember that a managed object context manages a number of records or managed objects. As a developer, you primarily interact with managed objects and the managed object context they belong to.
Why is a managed object context important? If you've read the tutorial about the Core Data stack, then you know that the persistent store coordinator bridges the gap between the persistent store and the managed object context. In the managed object context, records (managed objects) are created, updated, and deleted. Because the managed object context is unaware of the persistent store, it pushes its changes to the persistent store coordinator, which updates the persistent store.
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