django: add system check error for DecimalField (#400)

fixes #321

Author: timgraham

packages/django-google-spanner/django_spanner/base.py

@@ -14,6 +14,7 @@
 from .introspection import DatabaseIntrospection
 from .operations import DatabaseOperations
 from .schema import DatabaseSchemaEditor
+from .validation import DatabaseValidation
 
 
 class DatabaseWrapper(BaseDatabaseWrapper):
@@ -87,6 +88,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     introspection_class = DatabaseIntrospection
     ops_class = DatabaseOperations
     client_class = DatabaseClient
+    validation_class = DatabaseValidation
 
     @property
     def instance(self):

packages/django-google-spanner/django_spanner/validation.py

@@ -0,0 +1,21 @@
+import os
+
+from django.core import checks
+from django.db.backends.base.validation import BaseDatabaseValidation
+from django.db.models import DecimalField
+
+
+class DatabaseValidation(BaseDatabaseValidation):
+
+    def check_field_type(self, field, field_type):
+        errors = []
+        # Disable the error when running the Django test suite.
+        if os.environ.get('RUNNING_SPANNER_BACKEND_TESTS') != '1' and isinstance(field, DecimalField):
+            errors.append(
+                checks.Error(
+                    'DecimalField is not yet supported by Spanner.',
+                    obj=field,
+                    id='spanner.E001',
+                )
+            )
+        return errors