Foreign Key

Foreign-key relationship

One approach to solving the invalid semantic issue requires a common relationship called a foreign-key relationship. Creating a foreign key involves adding the ForeignKey argument to the Column creation. It requires that the field labeled as a foreign key in one table corresponds to the primary key in the other table. Setting nullable to False means that the BirthYear record must already exist. Without the foreign key, the database would allow the book record to have no corresponding birth_year reference. Whether to make it nullable or not depends on whether the linked-to record will already exist when the linked-from record is created.

In the following code, we’ve added a ForeignKey to Book: