django: fix power operator

The tests remain commented out because issue #331 also affects them.

fixes #333

Author: timgraham

django_spanner/features.py

@@ -257,8 +257,9 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # https://github.com/orijtech/django-spanner/issues/331
         'expressions.tests.ExpressionOperatorTests.test_lefthand_division',
         'expressions.tests.ExpressionOperatorTests.test_right_hand_division',
-        # power operator doesn't work:
-        # https://github.com/orijtech/django-spanner/issues/333
+        # power operator produces a float result, which can't be assigned to
+        # an integer column:
+        # https://github.com/orijtech/django-spanner/issues/331
         'expressions.tests.ExpressionOperatorTests.test_lefthand_power',
         'expressions.tests.ExpressionOperatorTests.test_righthand_power',
         # Cloud Spanner's docs: "The rows that are returned by LIMIT and OFFSET

django_spanner/operations.py

@@ -214,6 +214,8 @@ def format_for_duration_arithmetic(self, sql):
     def combine_expression(self, connector, sub_expressions):
         if connector == '%%':
             return 'MOD(%s)' % ', '.join(sub_expressions)
+        elif connector == '^':
+            return 'POWER(%s)' % ', '.join(sub_expressions)
         return super().combine_expression(connector, sub_expressions)
 
     def combine_duration_expression(self, connector, sub_expressions):