4. Create the `id` field:
- It could be `None` until the database assigns a value to it, so we annotate it with `Optional`.
+ It could be `None` until the database assigns a value to it, so we annotate it with `Optional` (`int | None` in Python 3.10+).
It is a **primary key**, so we use `Field()` and the argument `primary_key=True`.
In the database, the default value will be `NULL`, the SQL equivalent of `None`.
- As this field could be `None` (and `NULL` in the database), we annotate it with `Optional`.
+ As this field could be `None` (and `NULL` in the database), we annotate it with `Optional` (`int | None` in Python 3.10+).
8. Write the name of the database file.
9. Use the name of the database file to create the database URL.