Removing entries is an important aspect of database management and is used often in real-world applications. Users unsubscribe from services, products are removed from web applications, and some relationships are lost (unfollowing other users).

However, before we proceed, we need to be careful about one-to-many relationships. If we remove a reader, we would expect that all the reader’s reviews are also removed from our database. Similarly, removing a book should also remove all the reviews for that book. This procedure is called cascading deletion. Unfortunately, the way we previously declared our Reader and Book models will not perform the cascading deletion by default. To enable cascading deletions, we did a naive solution in this exercise by changing our models and re-initializing the database. In practice, database migration management is used to update a database schema.

To enable cascade deletions, we changed the models in the app.py by adding the cascade parameter to the .relationship() fields of Reader and Book models:

reviews = db.relationship('Review', backref='reviewer', lazy='dynamic', cascade = 'all, delete, delete-orphan')

In contrast, removing a review does not have any other cascading consequences on Book and Reader tables. Hence, specifying the cascading deletion option in Review is not needed.

Finally, to remove a reader with id = 753 we use the following command:

db.session.delete(Reader.query.get(753))

When you run playground.py you see that we print all the readers, all the reviews before and after the deletion. You can notice that when the reader with id = 753 is deleted, all their reviews are deleted as well. Refer to the image on the right to see the initial entries of some database tables.