added additional admin functionality

.gitignore

@@ -4,3 +4,6 @@
 *.un~
 /build
 /dist
+.DS_Store
+._*
+.svn

positions/__init__.py

@@ -1,2 +1,3 @@
 from positions.fields import PositionField
 from positions.managers import PositionManager
+from positions.admin import PositionAdmin

positions/admin.py

@@ -0,0 +1,45 @@
+from django.conf import settings
+from django.contrib import admin
+from django.shortcuts import get_object_or_404
+from django.utils.functional import update_wrapper
+from django.conf.urls.defaults import patterns, url
+from django.contrib.admin.util import unquote
+from django.http import HttpResponseRedirect
+
+
+class PositionAdmin(admin.ModelAdmin):
+    def get_urls(self):
+        def wrap(view):
+            def wrapper(*args, **kwargs):
+                return self.admin_site.admin_view(view)(*args, **kwargs)
+            return update_wrapper(wrapper, view)
+
+        info = self.model._meta.app_label, self.model._meta.module_name
+        return patterns('',
+            url(r'^(.+)/position-(up)/$', wrap(self.position_view), name='%s_%s_position_up' % info),
+            url(r'^(.+)/position-(down)/$', wrap(self.position_view), name='%s_%s_position_down' % info),
+        ) + super(PositionAdmin, self).get_urls()
+
+    def position_view(self, request, object_id, direction):
+        obj = get_object_or_404(self.model, pk=unquote(object_id))
+        if direction == 'up':
+            obj.position_up()
+        else:
+            obj.position_down()
+        return HttpResponseRedirect('../../')
+
+    def position_up_down_links(self, obj):
+        return '''
+        <a href="../../%(app_label)s/%(module_name)s/%(object_id)s/position-down/"><img src="%(STATIC_URL)spositions/img/arrow-up.gif" alt="Position Up" /></a>
+        <a href="../../%(app_label)s/%(module_name)s/%(object_id)s/position-up/"><img src="%(STATIC_URL)spositions/img/arrow-down.gif" alt="Position Down" /></a>''' % {
+            'app_label': self.model._meta.app_label,
+            'module_name': self.model._meta.module_name,
+            'object_id': obj.id,
+            'STATIC_URL': settings.STATIC_URL,
+        }
+
+    position_up_down_links.allow_tags = True
+
+    position_up_down_links.short_description = 'Position'
+
+

positions/fields.py

@@ -1,5 +1,6 @@
 import datetime
 import warnings
+import types
 
 from django.db import models
 from django.db.models.signals import post_delete, post_save, pre_delete
@@ -39,8 +40,8 @@ def __init__(self, verbose_name=None, name=None, default=-1, collection=None, pa
         self.collection = collection
         self.parent_link = parent_link
         self._collection_changed =  None
-
-    def contribute_to_class(self, cls, name):
+        
+    def contribute_to_class(self, cls, name ):
         super(PositionField, self).contribute_to_class(cls, name)
         for constraint in cls._meta.unique_together:
             if self.name in constraint:
@@ -50,6 +51,34 @@ def contribute_to_class(self, cls, name):
             if getattr(field, 'auto_now', False):
                 self.auto_now_fields.append(field)
         setattr(cls, self.name, self)
+
+        def _position_move(instance, up):
+            collection = self.get_collection(instance)
+            cache_name = self.get_cache_name()
+            position = getattr(instance, cache_name)[0]
+            if up:
+                collection = collection.filter(**{'%s__gt' % self.name: position})
+            else:
+                collection = collection.order_by('-%s' % self.name).filter(**{'%s__lt' % self.name: position})
+            try:
+                replacement = collection[0]
+            except IndexError:
+                return
+            current, updated = getattr(instance, cache_name), getattr(replacement, cache_name)
+            setattr(instance, cache_name, updated)
+            setattr(replacement, cache_name, current)
+            instance.save()
+            replacement.save()
+
+        def position_down(self):
+            return _position_move(self, up=False)
+
+        def position_up(self):
+            return _position_move(self, up=True)
+
+        setattr(cls, 'position_down', position_down )
+        setattr(cls, 'position_up', position_up )
+
         pre_delete.connect(self.prepare_delete, sender=cls)
         post_delete.connect(self.update_on_delete, sender=cls)
         post_save.connect(self.update_on_save, sender=cls)
@@ -167,8 +196,19 @@ def get_next_sibling(self, instance):
         Returns the next sibling of this instance.
         """
         try:
-            return self.get_collection(instance).filter(**{'%s__gt' % self.name: getattr(instance, self.get_cache_name())[0]})[0]
-        except:
+            position = getattr(instance, self.get_cache_name())[0]
+            return self.get_collection(instance).filter(**{'%s__gt' % self.name: position}).order_by('%s' % self.name)[0]
+        except IndexError:
+            return None
+
+    def get_previous_sibling(self, instance):
+        """
+        Returns the previous sibling of this instance.
+        """
+        try:
+            position = getattr(instance, self.get_cache_name())[0]
+            return self.get_collection(instance).filter(**{'%s__lt' % self.name: position}).order_by('-%s' % self.name)[0]
+        except IndexError:
             return None
 
     def remove_from_collection(self, instance):