Added SortableForeignKey field to replace sortable_by model property.

Refactored how the sortable_by properties get populated by looping over the model fields until we get to the SortableForeignKey, then grabbing properties from the field and its related data.

adminsortable/__init__.py

@@ -1,4 +1,4 @@
-VERSION = (1, 2, "f", 0)  # following PEP 386
+VERSION = (1, 3, "f", 0)  # following PEP 386
 DEV_N = None
 
 

adminsortable/admin.py

@@ -9,6 +9,7 @@ from django.shortcuts import render
 from django.template.defaultfilters import capfirst
 from django.views.decorators.csrf import csrf_exempt
 
+from adminsortable.fields import SortableForeignKey
 from adminsortable.models import Sortable
 
 STATIC_URL = settings.STATIC_URL
@@ -20,8 +21,17 @@ class SortableAdmin(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 __init__(self, *args, **kwargs):
         super(SortableAdmin, self).__init__(*args, **kwargs)
+
         self.has_sortable_tabular_inlines = False
         self.has_sortable_stacked_inlines = False
         for klass in self.inlines:
@@ -55,11 +65,16 @@ class SortableAdmin(ModelAdmin):
 
         #Determine if we need to regroup objects relative to a foreign key specified on the
         # model class that is extending Sortable.
-        sortable_by = getattr(self.model, 'sortable_by', None)
-        if sortable_by:
+        #Legacy support for 'sortable_by' defined as a model property
+        sortable_by_property = getattr(self.model, 'sortable_by', None)
+
+        #`sortable_by` defined as a SortableForeignKey
+        sortable_by_fk = self._get_sortable_foreign_key()
+
+        if sortable_by_property:
             #backwards compatibility for < 1.1.1, where sortable_by was a classmethod instead of a property
             try:
-                sortable_by_class, sortable_by_expression = sortable_by()
+                sortable_by_class, sortable_by_expression = sortable_by_property()
             except TypeError, ValueError:
                 sortable_by_class = self.model.sortable_by
                 sortable_by_expression = sortable_by_class.__name__.lower()
@@ -67,15 +82,24 @@ class SortableAdmin(ModelAdmin):
             sortable_by_class_display_name = sortable_by_class._meta.verbose_name_plural
             sortable_by_class_is_sortable = sortable_by_class.is_sortable()
 
+        elif sortable_by_fk:
+            #get sortable by properties from the SortableForeignKey field - supported in 1.3+
+            sortable_by_class_display_name = sortable_by_fk.rel.to._meta.verbose_name_plural
+            sortable_by_class = sortable_by_fk.rel.to
+            sortable_by_expression = sortable_by_fk.name.lower()
+            sortable_by_class_is_sortable = sortable_by_class.is_sortable()
+
+        else:
+            #model is not sortable by another model
+            sortable_by_class = sortable_by_expression = sortable_by_class_display_name =\
+            sortable_by_class_is_sortable = None
+
+        if sortable_by_property or sortable_by_fk:
             # Order the objects by the property they are sortable by, then by the order, otherwise the regroup
             # template tag will not show the objects correctly as
             # shown in https://docs.djangoproject.com/en/1.3/ref/templates/builtins/#regroup
             objects = objects.order_by(sortable_by_expression, 'order')
 
-        else:
-            sortable_by_class = sortable_by_expression = sortable_by_class_display_name =\
-            sortable_by_class_is_sortable = None
-
         try:
             verbose_name_plural = opts.verbose_name_plural.__unicode__()
         except AttributeError:

adminsortable/fields.py

@@ -0,0 +1,9 @@
+from django.db.models.fields.related import ForeignKey
+
+
+class SortableForeignKey(ForeignKey):
+    """
+    Field simply acts as a flag to determine the class to sort by.
+    This field replaces previous functionality where `sortable_by` was definied as a model property
+    that specified another model class.
+    """

adminsortable/models.py

@@ -1,6 +1,16 @@
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
 
+from adminsortable.fields import SortableForeignKey
+
+
+class MultipleSortableForeignKeyException(Exception):
+    def __init__(self, value):
+        self.value = value
+
+    def __str__(self):
+        return repr(self.value)
+
 
 class Sortable(models.Model):
     """
@@ -18,7 +28,10 @@ class Sortable(models.Model):
     Override `sortable_by` method to make your model be sortable by a foreign key field.
     Set `sortable_by` to the class specified in the foreign key relationship.
     """
+
     order = models.PositiveIntegerField(editable=False, default=1, db_index=True)
+
+    #legacy support
     sortable_by = None
 
     class Meta:
@@ -37,6 +50,17 @@ class Sortable(models.Model):
     def model_type_id(cls):
         return ContentType.objects.get_for_model(cls).id
 
+    def __init__(self, *args, **kwargs):
+        super(Sortable, self).__init__(*args, **kwargs)
+
+        #Validate that model only contains at most one SortableForeignKey
+        sortable_foreign_keys = []
+        for field in self._meta.fields:
+            if isinstance(field, SortableForeignKey):
+                sortable_foreign_keys.append(field)
+        if len(sortable_foreign_keys) > 1:
+            raise MultipleSortableForeignKeyException(u'%s may only have one SortableForeignKey' % self)
+
     def save(self, *args, **kwargs):
         if not self.id:
             try: