Moved logic to set sortable_foreign_key property to Sortable instead of SortableAdmin.

Added instance methods to get next or previous instance, optionally filtered by a sortable foreign key field.

adminsortable/admin.py

@@ -72,14 +72,6 @@ class SortableAdmin(SortableAdminBase, ModelAdmin):
     class Meta:
         abstract = True
 
-    def _get_sortable_foreign_key(self):
-        sortable_foreign_key = None
-        for field in self.model._meta.fields:
-            if isinstance(field, SortableForeignKey):
-                sortable_foreign_key = field
-                break
-        return sortable_foreign_key
-
     def get_urls(self):
         urls = super(SortableAdmin, self).get_urls()
         admin_urls = patterns('',
@@ -123,7 +115,7 @@ class SortableAdmin(SortableAdminBase, ModelAdmin):
         sortable_by_property = getattr(self.model, 'sortable_by', None)
 
         # `sortable_by` defined as a SortableForeignKey
-        sortable_by_fk = self._get_sortable_foreign_key()
+        sortable_by_fk = self.model.sortable_foreign_key
         sortable_by_class_is_sortable = get_is_sortable(objects)
 
         if sortable_by_property:

adminsortable/models.py

@@ -32,6 +32,7 @@ class Sortable(models.Model):
 
     # legacy support
     sortable_by = None
+    sortable_foreign_key = None
 
     class Meta:
         abstract = True
@@ -49,9 +50,13 @@ class Sortable(models.Model):
         for field in self._meta.fields:
             if isinstance(field, SortableForeignKey):
                 sortable_foreign_keys.append(field)
-        if len(sortable_foreign_keys) > 1:
+
+        sortable_foreign_keys_length = len(sortable_foreign_keys)
+        if sortable_foreign_keys_length > 1:
             raise MultipleSortableForeignKeyException(
                 u'{0} may only have one SortableForeignKey'.format(self))
+        elif sortable_foreign_keys_length == 1:
+            self.__class__.sortable_foreign_key = sortable_foreign_keys[0]
 
     def save(self, *args, **kwargs):
         if not self.id:
@@ -62,3 +67,28 @@ class Sortable(models.Model):
                 pass
 
         super(Sortable, self).save(*args, **kwargs)
+
+    def _filter_objects(self, filters, extra_filters):
+        if extra_filters:
+            filters.update(extra_filters)
+
+        if self.sortable_foreign_key:
+            # sfk_obj == sortable foreign key instance
+            sfk_obj = getattr(self, self.sortable_foreign_key.name)
+            filters.update(
+                {self.sortable_foreign_key.name: sfk_obj.id})
+
+        try:
+            obj = self._meta.model.objects.filter(**filters)[:1][0]
+        except IndexError:
+            obj = None
+
+        return obj
+
+    def get_next(self, extra_filters={}):
+        return self._filter_objects({'order__gt': self.order},
+            extra_filters)
+
+    def get_previous(self, extra_filters={}):
+        return self._filter_objects({'order__lt': self.order},
+            extra_filters)