Added a Post class which inherits from db.Model.
It represents blog posts, and its user_id field has been initialized as a foreign key.
The User class gets a field 'posts', which is a db.relationship field. For a one to many relationship, the db.relationship field is defined on the "one" side.
This relationship gives us a user.posts member from which we can list the posts of a particular user.
db.relationship(name of 'many' class, backref=field that will be added to objects of 'many' class that points back at the 'one' object or post.author, lazy).