Stop using sa-filters for basic grid sorting

this just breaks if we need to use "aliased" models e.g. when sorting
and/or filtering by Product "regular price" column and similar.  so
now sorting more like we always used to, except for multi-column.

nb. this still assumes callers use `Grid.make_sorter()` when declaring
the sorters.  if caller must specify more custom/explicit sort logic
then it likely will not work and we'll have to add a workaround to
allow avoiding the common logic..but that's another day

tailbone/grids/core.py

@@ -30,7 +30,6 @@ import logging
 
 import sqlalchemy as sa
 from sqlalchemy import orm
-from sa_filters import apply_sort
 
 from rattail.db.types import GPCType
 from rattail.util import prettify, pretty_boolean, pretty_quantity
@@ -1235,29 +1234,29 @@ class Grid(object):
         # TODO: is there a better way to check for SA sorting?
         if self.model_class:
 
-            # convert sort settings into a 'sortspec' for use with sa-filters
-            full_spec = []
+            # collect actual column sorters for order_by clause
+            sorters = []
             for sorter in self.active_sorters:
                 sortkey = sorter['field']
-                sortdir = sorter['order']
                 sortfunc = self.sorters.get(sortkey)
-                if sortfunc:
-                    spec = {
-                        'sortkey': sortkey,
-                        'model': sortfunc._class.__name__,
-                        'field': sortfunc._column.key,
-                        'direction': sortdir or 'asc',
-                    }
-                    full_spec.append(spec)
+                if not sortfunc:
+                    log.warning("unknown sorter: %s", sorter)
+                    continue
 
-            # apply joins needed for this sort spec
-            for spec in full_spec:
-                sortkey = spec['sortkey']
+                # join appropriate model if needed
                 if sortkey in self.joiners and sortkey not in self.joined:
                     data = self.joiners[sortkey](data)
                     self.joined.add(sortkey)
 
-            return apply_sort(data, full_spec)
+                # add column/dir to collection
+                sortdir = sorter['order']
+                sorters.append(getattr(sortfunc._column, sortdir)())
+
+            # apply sorting to query
+            if sorters:
+                data = data.order_by(*sorters)
+
+            return data
 
         else:
             # not a SQLAlchemy grid, custom sorter