Core Data - iOS Programming: The Big Nerd Ranch Guide - page 486

a painting might be of the Art asset type. BNRAssetType will be an entity in the model
file, and each row of that table will be mapped to an Objective-C object at runtime.
Add another entity called BNRAssetType to your model file. Give it an attribute called
label of type String . This will be the name of the category the BNRAssetType repres-
ents.
Figure 16.6 Create the BNRAssetType entity
Now, you need to establish relationships between BNRAssetType and BNRItem . Rela-
tionships between entities are mapped as pointers. There are two kinds of relationships:
to-many and to-one . When an entity has a to-one relationship, each instance of that entity
will have a pointer to an instance in the entity it has a relationship to. For example, the
BNRItem entity will have a to-one relationship to the BNRAssetType entity. Thus, a
BNRItem instance will have a pointer to a BNRAssetType instance.
When an entity has a to-many relationship, each instance of that entity has a pointer to an
NSSet . This set contains the instances of the entity that it has a relationship with. For ex-
ample, the BNRAssetType entity will have a to-many relationship to BNRItem because
many BNRItem s can have the same BNRAssetType . Thus, a BNRAssetType object
will have a set of all the BNRItem s that are its type of asset.
With these relationships, we can ask a BNRAssetType object for the set of BNRItem s
that fall into its category, and we can ask a BNRItem which BNRAssetType it falls un-
der. Figure 16.7 diagrams the relationships between BNRAssetType and BNRItem .
Figure 16.7 Entities in Homepwner