Introduction
Models are objects that represent the underlying data that your application presents to the user. Different apps will have very different models, depending on what problems they're trying to solve.
For example, a photo sharing application might have a Photo model to represent a particular photo, and a PhotoAlbum that represents a group of photos. In contrast, an online shopping app would probably have different models, like ShoppingCart, Invoice, or LineItem.
Models tend to be pe
