An idempotent migration to add an email uniqueness constraint

common/djangoapps/student/migrations/0011_auto_20170214_1200.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+# We used to have a uniqueness constraint on auth_user.email:
+# https://github.com/edx/edx-platform/commit/c52727b0e0fb241d8211900975d3b69fe5a1bd57
+#
+# That constraint was lost in the upgrade from Django 1.4->1.8.  This migration
+# adds it back.  But because it might already exist in databases created
+# long-enough ago, we have to do it idempotently.  So we check for the
+# existence of the constraint before creating it.
+
+def add_email_uniqueness_constraint(apps, schema_editor):
+    # Do we already have an email uniqueness constraint?
+    cursor = schema_editor.connection.cursor()
+    constraints = schema_editor.connection.introspection.get_constraints(cursor, "auth_user")
+    email_constraint = constraints.get("email", {})
+    if email_constraint.get("columns") == ["email"] and email_constraint.get("unique") == True:
+        # We already have the constraint, we're done.
+        return
+
+    # We don't have the constraint, make it.
+    schema_editor.execute("create unique index email on auth_user (email)")
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('student', '0010_auto_20170207_0458'),
+    ]
+
+    operations = [
+        migrations.RunPython(add_email_uniqueness_constraint)
+    ]