# history_tracking/mixins.py
from django.db import models
from django.conf import settings

class HistoricalChangeMixin(models.Model):
    """Mixin for historical models to track changes"""
    id = models.BigIntegerField(db_index=True, auto_created=True, blank=True)
    history_date = models.DateTimeField()
    history_id = models.AutoField(primary_key=True)
    history_type = models.CharField(max_length=1)
    history_user = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        null=True,
        on_delete=models.SET_NULL,
        related_name='+'
    )
    history_change_reason = models.CharField(max_length=100, null=True)
    
    class Meta:
        abstract = True
        ordering = ['-history_date', '-history_id']

    @property
    def prev_record(self):
        """Get the previous record for this instance"""
        try:
            return self.__class__.objects.filter(
                history_date__lt=self.history_date,
                id=self.id
            ).order_by('-history_date').first()
        except (AttributeError, TypeError):
            return None

    @property
    def diff_against_previous(self):
        prev_record = self.prev_record
        if not prev_record:
            return {}

        changes = {}
        for field in self.__dict__:
            if field not in [
                "history_date",
                "history_id",
                "history_type",
                "history_user_id",
                "history_change_reason",
                "history_type",
                "id",
                "_state",
                "_history_user_cache"
            ] and not field.startswith("_"):
                try:
                    old_value = getattr(prev_record, field)
                    new_value = getattr(self, field)
                    if old_value != new_value:
                        changes[field] = {"old": str(old_value), "new": str(new_value)}
                except AttributeError:
                    continue
        return changes

    @property
    def history_user_display(self):
        """Get a display name for the history user"""
        if hasattr(self, 'history_user') and self.history_user:
            return str(self.history_user)
        return None

    def get_instance(self):
        """Get the model instance this history record represents"""
        try:
            return self.__class__.objects.get(id=self.id)
        except self.__class__.DoesNotExist:
            return None