A Django foreign key that modifies None values to -1 so that the unique constraint in MySQL is respected.

unique_null.py

class UniqueNullForeignKey(models.ForeignKey):
    description = "Foreign Field that stores None as -1 to enforce the unique constraint."

    def from_db_value(self, value, expression, connection, context):
        """
        Intercepts the data being loaded and replaces -1 with None
        for proper use in the application.
        """
        return None if value == -1 else value

    def to_python(self, value):
        """
        Intercepts the data being loaded and replaces -1 with None
        for proper use in the application.
        """
        return None if value == -1 else value

    def get_prep_value(self, value):
        """
        Intercepts the data being saved and replaces None with -1
        so that the unique constraint holds.
        """
        return -1 if value is None else value

    def get_db_prep_save(self, value, connection):
        """
        Intercepts the data being saved and replaces None with -1
        so that the unique constraint holds.
        """
        return -1 if value is None else value