Assessing sqlalchemy.exc.ArguementError(…)

While undertaking a coding project earlier this week, all hell broke loose, not-so-literally. I was creating a User model, to store some information of registered users to the database, a Product model to store various details about the product and separate brand and categories models to data on brands and categories related to the product.

Upon setting up the models, I migrated the changes and upgraded the db’s version. A successful migration and upgrade process of course guaranteed that my models were all set and that there was absolutely nothing to worry about, or so I thought.

When firing up my application to perform random tests on its functionality, I ran into this error:

What followed was a moment of panic and a deep sense of dread, where could I have possibly gone wrong? Of course I now know better but that wasn’t the case then.

Upon further assessment on what could have triggered the issue, I noticed a rookie mistake but it was all-in-all quite an eye-opener! Let’s get to the crux of the matter:

Notice anything disturbing? Of course there exists a database relationship between brand-product and category-brand. Sqlalchemy allows us to express this relationship by use of db.relationship(). In a one-to-many relationship, db.relationship() is added on the one side and it takes the following as arguments in this case:

1. The model which is on the many side(don’t forget to capitalize the first character of this model)
2. The backref argument expresses how this relationship will be referenced. For example, if we wanted to find out what category a particular product belongs to, we’d simply reference product.category. Note that the db.relationship(…) is not a column in the table it has been listed in but it’s just a way that sqlalchemy allows us to create and reference relationships.
3. The lazy argument that implies how the query will be executed

The error was triggered by the fact that I used the same backref name on two separate occasions i.e in our example, the backref name ‘category’ was used twice. A simple name-change on either of the backrefs solves our problem and leaves our code error-free, at least for now.

Happy coding folks!