import six
[docs]
class ModelCachingDescriptor:
[docs]
def __init__(self, name, queryset):
self.name = name
self.id_name = f"_{name}_id"
self.object_name = f"_{name}_cache"
self.queryset = queryset
self.id_property = property(self.get_id, self.set_id)
self.object_property = property(self.get_object, self.set_object)
def _clear(self, instance):
setattr(instance, self.id_name, None)
setattr(instance, self.object_name, None)
[docs]
def set_id(self, instance, value):
if not value:
self._clear(instance)
elif isinstance(value, six.integer_types):
setattr(instance, self.id_name, value)
current_cached = self._get_cached_object(instance)
if current_cached and current_cached.pk != self.get_id(instance):
setattr(instance, self.object_name, None)
else:
raise TypeError(f"Error! Can't assign ID `{value!r}` in a `ModelCachingDescriptor({self.name})`.")
[docs]
def get_id(self, instance):
return getattr(instance, self.id_name, None)
[docs]
def set_object(self, instance, value):
if not value:
self._clear(instance)
elif isinstance(value, self.queryset.model):
if not value.pk:
raise ValueError(
f"Error! Can't assign unsaved model `{value!r}` in a `ModelCachingDescriptor({self.name})`."
)
setattr(instance, self.id_name, value.pk)
setattr(instance, self.object_name, value)
else:
raise TypeError(f"Error! Can't assign object `{value!r}` in a `ModelCachingDescriptor({self.name})`.")
[docs]
def get_object(self, instance):
if not self.get_id(instance):
return None
value = self._get_cached_object(instance)
if value is None:
value = self._cache_object(instance)
return value
def _cache_object(self, instance):
object = self.queryset.get(pk=self.get_id(instance))
setattr(instance, self.object_name, object)
setattr(instance, self.id_name, object.pk)
return object
def _get_cached_object(self, instance):
return getattr(instance, self.object_name, None)