Add grid filter which treats empty string as NULL

tailbone/grids/core.py

@@ -123,6 +123,9 @@ class Grid(object):
     def set_sorter(self, key, *args, **kwargs):
         self.sorters[key] = self.make_sorter(*args, **kwargs)
 
+    def set_filter(self, key, *args, **kwargs):
+        self.filters[key] = self.make_filter(key, *args, **kwargs)
+
     def set_label(self, key, label):
         self.labels[key] = label
         if key in self.filters:

tailbone/grids/filters.py

@@ -334,6 +334,24 @@ class AlchemyStringFilter(AlchemyGridFilter):
         ))
 
 
+class AlchemyEmptyStringFilter(AlchemyStringFilter):
+    """
+    String filter with special logic to treat empty string values as NULL
+    """
+
+    def filter_is_null(self, query, value):
+        return query.filter(
+            sa.or_(
+                self.column == None,
+                sa.func.trim(self.column) == ''))
+
+    def filter_is_not_null(self, query, value):
+        return query.filter(
+            sa.and_(
+                self.column != None,
+                sa.func.trim(self.column) != ''))
+
+
 class AlchemyByteStringFilter(AlchemyStringFilter):
     """
     String filter for SQLAlchemy, which encodes value as bytestring before