Question

I have 3 models.

class Picture(models.Model)
  name = models.CharField(max_length=255)
  image_field = models.ImageField(upload_to="foo/")
  slug = models.SlugField()
  [...]


class Size(models.Model):
  name = models.CharField(max_length=255)
  width = models.IntegerField()
  height = models.IntegerField()
  crop = models.BooleanField(default=True)
  upscale = models.BooleanField(default=False)

  def __unicode__(self):
    return self.name

class Cache(models.Model):
  size = models.ForeignKey('Size')
  picture = models.ForeignKey('Picture')
  image_field = models.ImageField(upload_to="picture/resize/")

I want to use them as follows: First generate Picture objects. Then create Size objects. For every Size and Picture a Cache object should be generated when needed.

My problem is that I don’t know where to put the code. It should be something like (pseudocode):

def get_cached_picture(Picture,Size):
  try:
    cacheObj = Cache.objects.get(picture=Picture, size=Size):
    [.. use cacheObj  ..]
  except Cache.DoesNotExist:
    [.. resize Picture according to Size, insert into cache, use it ..]

So where can I plug this code ? I know I could do this within a view, but is there a way to embed it into the models ? Cache should never be filled in the admin, instead it should be generated whenever a certain combination between Cache and Picture is needed.

It is probably easy to do, but I’m lacking of the right keyword for google.

Answer 1

Assumptions:

1. You want to be able to load a “Representation” of an image at a certain size on the fly.
2. Size model will store a predefined set of sizes that a Representation can have.

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I am going to refer to your Cache model as Representation because I think it makes more sense in context here.

The entry-point of this process would obviously be a view. A request comes in and you determine you need to search “Image1.jpg” @ 800×600. The simplest approach is just to put your queries right into the view. But for reusability, it might be better to do any of the following:

• A utility function which is then called from the view to retrieve a Representation object
• A classmethod of your Representation model, since you are already referencing Picture and Size as foreign keys.
• Using a custom manager on the Representation model to do something similar to the above

Any of these will make it reusable as an app if someone else were to have their own views and needed to simply retrieve a Rep instance directly.

While your pseudocode refers to this process being on the Picture mode, I maintain that it should probably be on the Representation model, since it contains both foreign keys and you can easily see if you have a proper rep. If not, create it.

The view should only have to call something simple as opposed to having all of the logic:

# as classmethod
rep = Representation.get_cached_picture(picture, size)
# or with a manager
rep = Representation.objects.get_cached_picture(picture, size)