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Introduction to Flask-SQLAlchemy
Creating database entries: relationships

Creating objects for tables that have foreign keys is not much different from the usual creation of Python objects.

Consider that we have the following objects already created:

b1 = Book(id = 123, title = 'Demian', author_name = 'Hermann', author_surname = 'Hesse') b2 = Book(id = 533, title = 'The stranger', author_name = 'Albert', author_surname = 'Camus') r1 = Reader(id = 342, name = 'Ann', surname = 'Adams', email = '') r2 = Reader(id = 312, name = 'Sam', surname = 'Adams', email = '')

To create an entry in the Review table, in addition to specifying a review text and a rating, we also need to specify which reader wrote the review, and for which book. In other words, we need to specify values for the review’s foreign keys reviewer_id and book_id that represent primary keys in Reader and Book, respectively.

rev1 = Review(id = 435, text = 'This book is amazing...', stars = 5, reviewer_id =, book_id =

In the example above we see that the review is written by Reader instance r1 for Book instance b1. We again used Python’s dot notation to access the id attribute of r1 and b1 objects.

Note: in the future, when creating database entries you don’t need to specify the primary key value explicitly, if you don’t have a preference for the values. When adding entries to a database, a primary key value will be automatically generated, unless specified. In the next lesson, we will see how to add entries to our database.



Create a new review instance called rev2 representing a review with id 450 given by the reader Sam Adams (object assigned to variable r2 for the book ‘The Stranger’ by Albert Camus (object assigned to variable b2). The review gives 2 stars and says ‘This book is difficult!’.


Print the number of words in the review object assigned to variable rev2.

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