fix on db to update visit_number for existing entries

smash/web/migrations/0057_update_visit_number_for_existing_visits.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.7 on 2017-10-27 10:22
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('web', '0056_visit_visit_number'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            "update web_visit s set visit_number= (select count(*) from web_visit t " +
+            "where t.subject_id=s.subject_id and t.datetime_begin<= s.datetime_begin);")
+    ]