Add __str__ to models, remove null=True in CharField and TextField

apps/logs/migrations/0002_replace_null_by_blank.py

@@ -0,0 +1,17 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('logs', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            "UPDATE logs_changelog SET previous = '' WHERE previous IS NULL;"
+        ),
+        migrations.RunSQL(
+            "UPDATE logs_changelog SET data = '' WHERE data IS NULL;"
+        ),
+    ]

apps/logs/migrations/0003_remove_null_tag_on_charfields.py

@@ -0,0 +1,23 @@
+# Generated by Django 2.2.16 on 2020-09-06 19:17
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('logs', '0002_replace_null_by_blank'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='changelog',
+            name='data',
+            field=models.TextField(blank=True, default='', verbose_name='new data'),
+        ),
+        migrations.AlterField(
+            model_name='changelog',
+            name='previous',
+            field=models.TextField(blank=True, default='', verbose_name='previous data'),
+        ),
+    ]

apps/logs/models.py

@@ -44,12 +44,14 @@ class Changelog(models.Model):
     )
 
     previous = models.TextField(
-        null=True,
+        blank=True,
+        default="",
         verbose_name=_('previous data'),
     )
 
     data = models.TextField(
-        null=True,
+        blank=True,
+        default="",
         verbose_name=_('new data'),
     )
 
@@ -80,3 +82,7 @@ class Changelog(models.Model):
     class Meta:
         verbose_name = _("changelog")
         verbose_name_plural = _("changelogs")
+
+    def __str__(self):
+        return _("Changelog of type \"{action}\" for model {model} at {timestamp}").format(
+            action=self.get_action_display(), model=str(self.model), timestamp=str(self.timestamp))

apps/logs/signals.py

@@ -99,7 +99,7 @@ def save_object(sender, instance, **kwargs):
             model = instance.__class__
             fields = changed_fields
 
-    previous_json = JSONRenderer().render(CustomSerializer(previous).data).decode("UTF-8") if previous else None
+    previous_json = JSONRenderer().render(CustomSerializer(previous).data).decode("UTF-8") if previous else ""
     instance_json = JSONRenderer().render(CustomSerializer(instance).data).decode("UTF-8")
 
     Changelog.objects.create(user=user,
@@ -149,6 +149,6 @@ def delete_object(sender, instance, **kwargs):
                              model=ContentType.objects.get_for_model(instance),
                              instance_pk=instance.pk,
                              previous=instance_json,
-                             data=None,
+                             data="",
                              action="delete"
                              ).save()