Question

I have a property in my model:

def _get_image(self):
    return Media.objects.get_for_object(self)

image = property(_get_image)

It calls the following function on my Media model:

def get_for_object(self, obj):
        ctype = ContentType.objects.get_for_model(obj)
        return self.filter(items__content_type__pk=ctype.pk, items__object_id=obj.pk)

Then in my template I am iterating through the results like so:

{% if entry.image %}
<h2>Current image:</h2>
{% for m in entry.image %}
    {{ m }}
{% endfor %}
{% endif %}

For some reason, my SQL readout shows these two queries, right next to each other:

0.40    SELECT
EXPLAIN
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SELECT `media_media`.`id`, `media_media`.`file`, `media_media`.`content_type`, `media_media`.`created` FROM `media_media` INNER JOIN `media_mediaattachment` ON (`media_media`.`id` = `media_mediaattachment`.`media_id`) WHERE (`media_mediaattachment`.`content_type_id` = 12 AND `media_mediaattachment`.`object_id` = 20 )
0.38    SELECT
EXPLAIN
Toggle Stacktrace
SELECT `media_media`.`id`, `media_media`.`file`, `media_media`.`content_type`, `media_media`.`created` FROM `media_media` INNER JOIN `media_mediaattachment` ON (`media_media`.`id` = `media_mediaattachment`.`media_id`) WHERE (`media_mediaattachment`.`content_type_id` = 12 AND `media_mediaattachment`.`object_id` = 20 )

So whenever I access entry.image, the database is getting hit. Surely it should store the results or something?

Answer 1

Here’s how you write a caching property without explicitly setting the cache to None in the __init__ method:

def _get_image(self):
    if not hasattr(self, '_image'):
        self._image = Media.objects.get_for_object(self)
    return self._image

image = property(_get_image)

or in more modern syntax

@property
def image(self):
    if not hasattr(self, '_image'):
        self._image = Media.objects.get_for_object(self)
    return self._image