Django: changing model's schema (replace CharField with ImageField) - django

Question

I modify project based on Django framework. I have form to add an item. Item has the cover (an image). Current version of model for this item store cover's url like this:
class Item(models.Model):
title = models.CharField(max_length = 255, db_index = True)
slug = models.CharField(max_length = 80, db_index = True)
categories = models.ManyToManyField(Category)
cover_url = models.CharField(max_length = 255, null = True, default = None)
...
Important notice that some images stored on other servers (different file hosting).
I want to replace CharField with ImageField. But how about existing items? I want to change model's schema and save all previously added images. How I can achieve this goal?
Maybe some reasons for this modification can be helpful. The main reason is to provide to users ability to upload images from their computer (not only insert the urls as it was).
TIA!

Answer 1

If cover_url can have existing source - you must have custom storage, that can handle external sources.
Here is example of custom storage usage for ImageField from django documentation:
from django.db import models
from django.core.files.storage import FileSystemStorage
fs = FileSystemStorage(location='/media/photos')
class Car(models.Model):
...
photo = models.ImageField(storage=fs)
Let's jump out of it and we will get code like this:
from django.db import models
from django.core.files.storage import FileSystemStorage
def is_url(name):
return 'http' in name
class MyStorage(FileSystemStorage):
#We should override _save method, instead of save.
def _save(self, name, content=None):
if content is None and is_url(name):
return name
super(MyStorage, self)._save(name, content)
fs = MyStorage()
class Item(models.Model):
title = models.CharField(max_length = 255, db_index = True)
slug = models.CharField(max_length = 80, db_index = True)
categories = models.ManyToManyField(Category)
cover_url = models.ImageField(storage=fs)
It has big room for improvements - here is shown only idea.