# custom_sessions.py

from django.contrib.sessions.backends.db import SessionStore as DbSessionStore
from django.contrib.sessions.base import CreateError
from django.core.exceptions import SuspiciousOperation
from django.db import IntegrityError, transaction
from django.db.models import F, ExpressionWrapper, fields
from django.utils import timezone

class CustomDbSessionStore(DbSessionStore):
    """
    A custom database-backed session store that extends the default Django
    database-backed session store.
    """

    def create_model_instance(self, data, session_key=None, expire_date=None):
        """
        Create a new session model instance.

        This method overrides the default implementation to include additional
        fields or customization.

        Args:
            data (dict): The session data to be stored.
            session_key (str): The session key.
            expire_date (datetime): The expiration date of the session.

        Returns:
            The newly created session model instance.
        """
        if session_key is None:
            # Generate a new session key
            session_key = self._get_new_session_key()

        # Create the model instance with additional fields
        s = self.model(
            session_key=session_key,
            session_data=self.encode(data),
            expire_date=expire_date or self.get_expiry_date(),
            # Add your additional fields here
            custom_field="example",
        )

        # Use atomic transaction to avoid race conditions
        with transaction.atomic():
            # Try to save the session
            try:
                s.save(force_insert=True)
            except IntegrityError:
                # If the session key already exists, generate a new one
                if session_key is not None:
                    session_key = self._get_new_session_key()
                    s.session_key = session_key
                    s.save(force_insert=True)
                else:
                    raise CreateError

        return s

    def clean_up(self):
        """
        Clean up expired sessions.

        This method overrides the default implementation to include additional
        cleanup logic.

        Returns:
            The number of sessions cleaned up.
        """
        # Use atomic transaction to avoid race conditions
        with transaction.atomic():
            # Delete sessions that have expired
            expired_sessions = self.model.objects.filter(expire_date__lt=timezone.now())
            count = expired_sessions.delete()[0]

        return count

# settings.py

SESSION_ENGINE = 'your_app.custom_sessions.CustomDbSessionStore'