fix django warning

src/c3nav/mapdata/migrations/0080_auto_20191224_2203.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.8 on 2019-12-24 21:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mapdata', '0079_auto_20191224_1858'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='report',
+            name='created_groups',
+            field=models.ManyToManyField(blank=True, help_text='select all groups that apply, if any', limit_choices_to={'can_report_missing': True}, related_name='_report_created_groups_+', to='mapdata.LocationGroup', verbose_name='location groups'),
+        ),
+    ]

src/c3nav/mapdata/models/report.py

@@ -81,7 +81,7 @@ class Report(models.Model):
                               help_text=_('you have to supply a title in at least one language'))
     created_groups = models.ManyToManyField('mapdata.LocationGroup', verbose_name=_('location groups'), blank=True,
                                             limit_choices_to={'can_report_missing': True},
-                                            help_text=_('select all groups that apply, if any'))
+                                            help_text=_('select all groups that apply, if any'), related_name='+')
     secret = models.CharField(_('secret'), max_length=32, default=get_report_secret)
 
     coordinates = LocationById()