Django: Why cannot I use the custom file field?

[imhomer]

I have been tortured by this problem for a whole week. I really hope a superman can help me solve it. I tried to create a new file field called "ThumbnailImage Field". The following is my model.py:

from django.db import models
from django.contrib import admin
from cn.han import ThumbnailImageField
from django.db.models.fields.files import ImageField, ImageFieldFile
# Create your models here.

class Item(models.Model):
    name = models.CharField(max_length=250)
    description = models.TextField()

    class Meta:
        ordering = ['name']

    def __unicode__(self):
        return self.name

    @models.permalink
    def get_absolute_url(self):
        return ('item_detail', None, {'object_id': self.id})

class Photo(models.Model):
    item = models.ForeignKey(Item)
    title = models.CharField(max_length=100)
    image = ThumbnailImageField(upload_to='photos')
    #TypeError here: 'module' object is not callable.
    caption = models.CharField(max_length=250, blank=True)

    class Meta:
        ordering = ['title']

    def __unicode__(self):
        return self.title

    @models.permalink
    def get_absolute_url(self):
        return ('photo_detail', None, {'object_id': self.id})

class PhotoInline(admin.StackedInline):
    model = Photo

class ItemAdmin(admin.ModelAdmin):
    inlines = [PhotoInline]

admin.site.register(Item, ItemAdmin)
admin.site.register(Photo)

The following is my ThumnailImageFi eld.py which stored under /cn/han/:

from django.db.models.fields.files import ImageField, ImageFieldFile
from PIL import Image
import os
from django.db import models

def _add_thumb(s):
    """
    Modifies a string (filename, URL) containing an image filename, to insert
    '.thumb' before the file extension (which is changed to be '.jpg').
    """
    parts = s.split(".")
    parts.insert(-1, "thumb")
    if parts[-1].lower() not in ['jpeg', 'jpg']:
        parts[-1] = 'jpg'
    return ".".join(parts)

class ThumbnailImageField(ImageField):
    """
    Behaves like a regular ImageField, but stores an extra (JPEG) thumbnail
    image, providing get_FIELD_thumb_url() and get_FIELD_thumb_filename().

    Accepts two additional, optional arguments: thumb_width and thumb_height,
    both defaulting to 128 (pixels). Resizing will preserve aspect ratio while
    staying inside the requested dimensions; see PIL's Image.thumbnail()
    method documentation for details.
    """
    
    def __init__(self, thumb_width=128, thumb_height=128, *args, **kwargs):
        self.thumb_width = thumb_width
        self.thumb_height = thumb_height
        super(ThumbnailImageField, self).__init__(*args, **kwargs)

    def _get_path(self):
        self._require_file()
        return self.storage.path(self.name)
    path = property(_get_path)
    
    class ThumbnailImageFieldFile(ImageFieldFile):
        def _get_thumb_path(self):
            return _add_thumb(self.path)
        thumb_path = property(_get_thumb_path)

        def _get_thumb_url(self):
            return _add_thumb(self.url)
        thumb_url = property(_get_thumb_url)

        def save(self, name, content, save=True):
            super(ThumbnailImageFieldFile, self).save(name, content, save)
            img = Image.open(self.path)
            img.thumbnail(
                (self.field.thumb_width, self.field.thumb_height),
                Image.ANTIALIAS
            )
            img.save(self.thumb_path, 'JPEG')

        def delete(self, save=True):
            if os.path.exists(self.thumb_path):
                os.remove(self.thumb_path)
            super(ThumbnailImageFieldFile, self).delete(save)
    attr_class = ThumbnailImageFieldFile

Finally here is my template called items_list.html :

{% extends "base.html" %}

{% block title %}Item List{% endblock %}

{% block content %}

<p><a href="{% url index %}">&laquo; Back to main page</a></p>

<h2>Items</h2>
{% if object_list %}
<table>
    <tr>
        <th>Name</th>
        <th>Sample Thumb</th>
        <th>Description</th>
    </tr>
    {% for item in object_list %}
    <tr>
        <td><i>{{ item.name }}</i></td>
        <td>
            {% if item.photo_set.count %}
            <a href="{{ item.get_absolute_url }}">
               <img src="{{ item.photo_set.all.0.image.thumb_url }}" />
            </a>
            {% else %}
            (No photos currently uploaded)
            {% endif %}
        </td>
        <td>{{ item.description }}</td>
    </tr>
    {% endfor %}
</table>
{% else %}
<p>There are currently no items to display.</p>
{% endif %}

{% endblock %}