Data Model
Core Data describes the data in terms of a data model. A data model is used to define the structure of the data in terms of entities, properties, and their relationships.
Entities
Because Core Data maintains data in terms of objects, an entity is an individual data object to represent complete information of the person, item, object, and so on. For example, customer is an entity, which represents information of customers, such as name, address, e-mail ID, contact number, products purchased, date of purchase, and so on. Similarly, the product is an entity, which represents the information of a product, such as name of the product, price, weight, and so on. An entity consists of properties that are a combination of attributes and relationships. An entity in Xcode's Data Model Editor may appear as shown in the following screenshot:
Properties
Properties of an entity give detailed information about it, such as what are its attributes and how it is related to other entities. A property of an entity refers to its attributes and relationships. Attributes are scalar values and relationships are pointers to or collections of other entities at the object level. A property is represented by a name and a type.
Attributes
Attributes are the variables within an object (entity). In fact, a collection of attributes makes an entity. In database language, they are known as columns of the table. For example, the customer's entity may consist of attributes such as name, address, contact number, items purchased, and so on. Similarly, the attributes in the products table may be item code, item name, quantity, and so on. While creating attributes of an entity, we have to specify its name and its data type to declare the kind of information (whether integer, float, string, and so on) that will be stored in the attribute. Also, we can define the constraints on the information that can be stored in the column. For example, we can specify the maximum, minimum value (range) that can be stored in that attribute, or whether the attribute can or cannot store certain special symbols, and so on. Also, we can specify the default value of an attribute.
Relationships
Besides attributes, an entity may also contain relationships (which define how an entity is related to other entities). The attributes and relationships of an entity are collectively known as properties. The relationships are of many types (To-One, To-Many, and Many-to-Many) and play a major role in defining connection among the entities and what will be the impact of insertion or deletion of a row in one entity on the connected entities.
Examples of relationship types:
- The relationship from a child entity to a parent entity is a To-One relationship as a child can have only one parent
- The relationship from a customer to a product entity is a To-Many relationship as a customer can purchase several products
- The relationship from an employee to a project entity is of Many-to-Many type as several employees can work on one project and an employee can work on several projects simultaneously
Note
To define a many-to-many relationship in Core Data, we have to use two To-many relationships. The first To-many relationship is set from the first entity to the second entity. The second To-many relationship is set from the second entity to the first entity.
In Xcode's Data Model Editor, the relationship from Customer to Product—a To-Many relationship—is represented by a line that appears pointing from the Customer entity to the Product entity with two arrows, (designating a One-to-Many relationship) as shown in the subsequent screenshot, whereas the To-One relationship is represented by a line with a single arrow:
When defining relationships in Core Data we may use inverse relationships, though it's optional.
Inverse relationship
In Core Data, every relationship can have an inverse relationship. Like, if there is a relationship from Customer to Product, there will be a relationship from Product to Customer too. A relationship does not need to be the same kind as its inverse; for example, a To-One relationship can have an inverse relationship of type To-Many. Although relationships are not required to have an inverse, Apple generally recommends that you always create and specify the inverse, (even if you won't need) as it helps Core Data to ensure data integrity.
For example, consider a situation when a Customer entity has a relationship of the To-Many type to a Product entity and some information of a customer is changed or a row of a customer is deleted. Then it will be easier for Core Data to ensure consistency; that is, by inverse relationship, Core Data can automatically find the products related to the deleted customer and hence, delete them too. Inverse relationship is practically used in Chapter 8, Entering, Saving, Listing, and Deleting the Records of the Products Sold to the Customers.
Before we go further, let us have a quick look at the architecture that is used in iPhone application development: MVC.