fix #5145 bug(reporting): Model Changes to allow old_status to be None and str rep change (#5146)

Because:

* old_status needs to be ble to be None for creation changelogs
* String representation change would allow for easier debugging

This commit:

* allows old_status to be None
* Changes the str rep of ReportLog object to something more descriptive

app/experimenter/reporting/migrations/0002_auto_20210502_0608.py

@@ -0,0 +1,31 @@
+# Generated by Django 3.1.8 on 2021-05-02 06:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("reporting", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="reportlog",
+            name="experiment_old_status",
+            field=models.CharField(
+                blank=True,
+                choices=[
+                    ("Draft", "Draft"),
+                    ("Preview", "Preview"),
+                    ("Review", "Review"),
+                    ("Ship", "Ship"),
+                    ("Accepted", "Accepted"),
+                    ("Live", "Live"),
+                    ("Complete", "Complete"),
+                ],
+                max_length=255,
+                null=True,
+            ),
+        ),
+    ]

app/experimenter/reporting/models.py

@@ -16,7 +16,10 @@ class ReportLog(models.Model):
         choices=ReportLogConstants.ExperimentType.choices,
     )
     experiment_old_status = models.CharField(
-        max_length=255, choices=ReportLogConstants.ExperimentStatus.choices
+        max_length=255,
+        choices=ReportLogConstants.ExperimentStatus.choices,
+        blank=True,
+        null=True,
     )
     experiment_new_status = models.CharField(
         max_length=255, choices=ReportLogConstants.ExperimentStatus.choices
@@ -40,6 +43,9 @@ class ReportLog(models.Model):
         self.clean()
         super().save(*args, **kwargs)
 
+    def __str__(self):
+        return f"{self.timestamp} - {self.experiment_type} - {self.event_reason} "
+
     class Meta:
         ordering = ("timestamp",)
         verbose_name = "ReportLog"

app/experimenter/reporting/tests/test_models.py

@@ -15,3 +15,15 @@ class TestReportLog(TestCase):
         self.assertEqual(ReportLog.objects.count(), 0)
         ReportLogFactory.create()
         self.assertEqual(ReportLog.objects.count(), 1)
+
+    def test_reportlog_saves_with_null_old_status(self):
+        self.assertEqual(ReportLog.objects.count(), 0)
+        ReportLog.objects.create(
+            experiment_slug="experiment-slug",
+            experiment_name="experiment name",
+            experiment_type="Nimbus IOS",
+            experiment_new_status="Draft",
+            event="Create",
+            event_reason="New",
+        )
+        self.assertEqual(ReportLog.objects.count(), 1)