avoid errors when forward-migrating course_groups after south conversion

JIRA: TNL-937

openedx/core/djangoapps/course_groups/migrations/0001_initial.py

@@ -2,13 +2,30 @@
 import datetime
 from south.db import db
 from south.v2 import SchemaMigration
-from django.db import models
+from django.db import models, connection
 
 
 class Migration(SchemaMigration):
 
     def forwards(self, orm):
         # Adding model 'CourseUserGroup'
+
+        def table_exists(name):
+            return name in connection.introspection.table_names()
+
+        def index_exists(table_name, column_name):
+            return column_name in connection.introspection.get_indexes(connection.cursor(), table_name)
+
+        # Since this djangoapp has been converted to south migrations after-the-fact,
+        # these tables/indexes should already exist when migrating an existing installation.
+        if not (
+            table_exists('course_groups_courseusergroup') and
+            index_exists('course_groups_courseusergroup', 'name') and
+            index_exists('course_groups_courseusergroup', 'course_id') and
+            table_exists('course_groups_courseusergroup_users') and
+            index_exists('course_groups_courseusergroup_users', 'courseusergroup_id') and
+            index_exists('course_groups_courseusergroup_users', 'user_id')
+        ):
             db.create_table('course_groups_courseusergroup', (
                 ('id', self.gf('django.db.models.fields.AutoField')(primary_key=True)),
                 ('name', self.gf('django.db.models.fields.CharField')(max_length=255)),