Allow a content class to extend another.
- Content class Product has 3 attributes : a title, a description and a price ;
- Content class Book extends Product. It has one more attribute : number of page ;
- Content class DVD extends Product. It has one more attribute : time.
If I add attributes to the content class Product, It is automaticaly added to Book & DVD.
I can fetch my Books or my DVDs. If I fecth my Products, I will have my Books and my DVDs together.