Fixed #15697 -- Made sqlindexes aware of auto-created tables

Thanks mbertheau for the report and Ash Christopher for the
initial patch.

tests/commands_sql/models.py

@@ -2,6 +2,12 @@ from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
 
+@python_2_unicode_compatible
+class Comment(models.Model):
+    pass
+
+
 @python_2_unicode_compatible
 class Book(models.Model):
     title = models.CharField(max_length=100, db_index=True)
+    comments = models.ManyToManyField(Comment)