from django.core.cache.backends.base import BaseCache
from django.db import transaction
from myapp.models import CacheEntry
 
class DatabaseCacheBackend(BaseCache):
    def __init__(self, params):
        super().__init__(params)
        self.version = params.get('VERSION', 1)
    
    def make_key(self, key, version=None):
        version = version or self.version
        return f"cache:v{version}:{key}"
    
    def add(self, key, value, timeout=None):
        return self._set_or_get(key, value, timeout, add=True) is None
    
    def _set_or_get(self, key, value=None, timeout=None, add=False):
        cache_key = self.make_key(key)
        expires_at = self._get_expires_at(timeout)
        try:
            with transaction.atomic():
                entry = CacheEntry.objects.filter(cache_key=cache_key).first()
                if entry:
                    if add:
                        return entry
                    entry.value = self.serialize(value)
                    entry.expires_at = expires_at
                    entry.save()
                else:
                    CacheEntry.objects.create(
                        cache_key=cache_key,
                        value=self.serialize(value),
                        expires_at=expires_at
                    )
        except Exception:
            return None
        return None
 
    def get(self, key, default=None):
        cache_key = self.make_key(key)
        try:
            entry = CacheEntry.objects.get(cache_key=cache_key)
            if entry.is_expired:
                entry.delete()
                return default
            return self.deserialize(entry.value)
        except CacheEntry.DoesNotExist:
            return default