I started learning 📖 Django few months back, and this is a practice project from MDN Web Docs that touches the aspects of Django relatively broadly.

This project is about an effort to take a small library online, hence the name locallibrary.

Before we jump in and start coding the models, it's worth taking a few minutes to think about what data we need to store and the relationships between the different objects.

We know that we need to store information about books (title, summary, author, written language, category, ISBN) and that we might have multiple copies available (with globally unique id, availability status, etc.). We might need to store more information about the author than just their name, and there might be multiple authors with the same or similar names. We want to be able to sort information based on book title, author, written language, and category.

When designing our models it makes sense to have separate models for every "object" (a group of related information). In this case, the obvious objects are books, book instances(to uniquely identify a book in the system), and authors.

We might also want to use models to represent selection-list options (e.g. like a drop down list of choices), rather than hard coding the choices into the website itself — this is recommended when all the options aren't known up front or may change. Obvious candidates for models, in this case, include the book genre (e.g. Science Fiction, French Poetry, etc.) and language (English, French, Japanese).

Once we've decided on our models and field, we need to think about the relationships. Django allows us to define relationships that are one to one (OneToOneField), one to many (ForeignKey) and many to many (ManyToManyField).

With that in mind, the UML association diagram below shows the models we'll define in this case (as boxes).

We've created models for the book (the generic details of the book), book instance (status of specific physical copies of the book available in the system), and author. We have also decided to have a model for the genre so that values can be created/selected through the admin interface. We've decided not to have a model for the BookInstance:status — we've hardcoded the values (LOAN_STATUS) because we don't expect these to change. Within each of the boxes, you can see the model name, the field names, and types, and also the methods and their return types.

The diagram also shows the relationships between the models, including their multiplicities. The multiplicities are the numbers on the diagram showing the numbers (maximum and minimum) of each model that may be present in the relationship. For example, the connecting line between the boxes shows that Book and a Genre are related. The numbers close to the Genre model show that a book must have one or more Genres (as many as you like), while the numbers on the other end of the line next to the Book model show that a Genre can have zero or many associated books.