make Log.transaction nullable and change ordering

zoo/zoo_data/migrations/0004_auto_20170302_1559.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.4 on 2017-03-02 15:59
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zoo_data', '0003_auto_20170104_1857'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='log',
+            options={'ordering': ('-timestamp', '-id'), 'verbose_name': 'log', 'verbose_name_plural': 'logs'},
+        ),
+        migrations.AlterField(
+            model_name='log',
+            name='transaction',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='zoo_data.Transaction', verbose_name='transaction'),
+        ),
+        migrations.AlterField(
+            model_name='relation',
+            name='left',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='left_relations', to='zoo_data.Entity', verbose_name='left'),
+        ),
+        migrations.AlterField(
+            model_name='relation',
+            name='right',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='right_relations', to='zoo_data.Entity', verbose_name='right'),
+        ),
+    ]

zoo/zoo_data/models.py

@@ -172,6 +172,7 @@ class Log(models.Model):
         verbose_name=_('entity'))
     transaction = models.ForeignKey(
         'Transaction',
+        null=True,
         verbose_name=_('transaction'))
     timestamp = models.DateTimeField(
         auto_now_add=True,
@@ -187,6 +188,6 @@ class Log(models.Model):
         verbose_name=_('url'))
 
     class Meta:
-        ordering = ('timestamp',)
+        ordering = ('-timestamp', '-id')
         verbose_name = _('log')
         verbose_name_plural = _('logs')