Django models - problem importing

Question

I've refactored my models files into a module - this way it's much easier to maintain the code since it has grown quite a bit.

The funny thing is though that it won't work for one of the classes that references another class that references the fist one in it's turn:

UPD: the cycling references are confusing python and this is what the problem is caused by. This is easy to fix when you only reference other models from your model definition. However, Picture has methods that reference paperType class and vice versa - how can this be fixed?

Here's class Picture:

from django.db import models
from django.utils import simplejson
from picviewer.models import Collection, ImageSizeRatio, printSize

class Picture(models.Model):
    name = models.TextField(null=False,blank=False,unique=False)
    collection = models.ForeignKey(Collection)
    popularity = models.IntegerField(default=0,unique=False)
    isPurchasable = models.BooleanField(default=False)
    allowBuyExclusive = models.BooleanField(default=False)
    basePrice = models.DecimalField(decimal_places=2,max_digits=8)
    imageSizeRatio = models.ForeignKey(ImageSizeRatio)
    imageThumbnail = models.FileField(upload_to='pictures')
    imagePreview = models.FileField(upload_to='pictures')
    imageSmall = models.FileField(upload_to='pictures')
    imageNormal = models.FileField(upload_to='pictures')
    imageLarge = models.FileField(upload_to='pictures')
    imageHuge = models.FileField(upload_to='pictures')
    allowedPrintSize = models.ManyToManyField(printSize)

Here is printSize class that it references - you see it calls Picture functions to do some math around pictures of specified printSize:

from django.db import models
from picviewer.models import paperType
from picviewer.models import Picture

class printSize (models.Model):
    name = models.CharField(null=False,blank=False,unique=True,max_length=60)
    width = models.IntegerField(null=False,blank=False)
    height = models.IntegerField(null=False,blank=False)
    allowedPaperType = models.ManyToManyField(paperType)
    #isActive = models.NullBooleanField(null=True, default=None)

    def json(self, picture_id, base_price):
        sizes_selector = printSize.objects.filter(picture__id = picture_id)
        sizes = list()
        for size in sizes_selector:
            papers = list()
            for paper in size.allowedPaperType.all():
                cost_for_paper = Picture.objects.get(id=picture_id).calculatePrice(paper.id,size.id)
                p = dict(id = paper.id,
                         name = paper.name,
                         description = paper.description,
                         price = paper.pricePerSqMeter.__str__(),
                         cost = "%.2f" % cost_for_paper)
                papers.append(p)
            s = dict(id = size.id,
                     name = size.name,
                     width = size.width,
                     height = size.height,
                     allowedPapers = papers)
            sizes.append(s)
        return sizes

now this is what I get in shell trying to import Picture:

>>> from picviewer.models import Picture
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "D:\~Sasha\eclipse_workspace\zavalen\picviewer\models\Picture.py", line 4, in <module>
    from picviewer.models import Collection, ImageSizeRatio, printSize
  File "D:\~Sasha\eclipse_workspace\zavalen\picviewer\models\printSize.py", line 4, in <module>
    from picviewer.models import Picture
ImportError: cannot import name Picture
>>>

can I cure this? :)

Answer 1

To avoid cyclic imports, specify FK model as a string, e.g

collection = models.ForeignKey('Collection') # Collection is in the same module

or

collection = models.ForeignKey('myapp.Collection') # Collection is in another app