Often times in real-world applications we will have entities that are somehow related. Students take courses, customers buy products, and users comment on posts. In SQLAlchemy we can declare a relationship with a field initialized with the .relationship() method. In one-to-many relationships, the relationship field is used on the ‘one’ side of the relationship. In our use case we have the following one-to-many relationships:

1. One book ———< many reviews for that book
2. One reader ——–< many reviews from that reader

Hence, we add relationship fields to the Book and Reader models. In this exercise, we will show you how to add a relationship to the Book model, and you will do the same for the Reader model.

We declare a one-to-many relationship between Book and Review by creating the following field in the Book model:

reviews = db.relationship('Review', backref='book', lazy='dynamic')

where

• the first argument denotes which model is to be on the ‘many’ side of the relationship: Review.
• backref = 'book' establishes a book attribute in the related class (in our case, class Review) which will serve to refer back to the related Book object.
• lazy = dynamic makes related objects load as SQLAlchemy’s query objects.

By adding relationship to Book we only handled one side in our one-to-many relationship. Specifically, we only covered the direction denoted by the red arrow in the schema below:

In the next exercise, we will add the Review model and its relationship with the Book model (the blue arrow).