Collection main_item: make a OneToOneField intsead of just a foreign key.

django_etebase/migrations/0030_auto_20200922_0832.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.1.1 on 2020-09-22 08:32
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etebase', '0029_auto_20200907_0801'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collection',
+            name='main_item',
+            field=models.OneToOneField(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='parent', to='django_etebase.collectionitem'),
+        ),
+    ]