diff --git a/django_etebase/migrations/0017_auto_20200623_0958.py b/django_etebase/migrations/0017_auto_20200623_0958.py
new file mode 100644
index 0000000..e244b13
--- /dev/null
+++ b/django_etebase/migrations/0017_auto_20200623_0958.py
@@ -0,0 +1,25 @@
+# Generated by Django 3.0.3 on 2020-06-23 09:58
+
+import django.core.validators
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_etebase', '0016_auto_20200623_0820'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='collection',
+            name='main_item',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='parent', to='django_etebase.CollectionItem'),
+        ),
+        migrations.AlterField(
+            model_name='collectionitem',
+            name='uid',
+            field=models.CharField(db_index=True, max_length=43, validators=[django.core.validators.RegexValidator(message='Not a valid UID', regex='^[a-zA-Z0-9]*$')]),
+        ),
+    ]
diff --git a/django_etebase/models.py b/django_etebase/models.py
index 2702389..87b0ed1 100644
--- a/django_etebase/models.py
+++ b/django_etebase/models.py
@@ -58,7 +58,7 @@ class Collection(models.Model):
 
 
 class CollectionItem(models.Model):
-    uid = models.CharField(db_index=True, blank=False, null=True,
+    uid = models.CharField(db_index=True, blank=False,
                            max_length=43, validators=[UidValidator])
     collection = models.ForeignKey(Collection, related_name='items', on_delete=models.CASCADE)
     version = models.PositiveSmallIntegerField()
diff --git a/django_etebase/views.py b/django_etebase/views.py
index 4f5d757..2e4bf3c 100644
--- a/django_etebase/views.py
+++ b/django_etebase/views.py
@@ -228,7 +228,6 @@ class CollectionItemViewSet(BaseViewSet):
             raise Http404("Collection does not exist")
         # XXX Potentially add this for performance: .prefetch_related('revisions__chunks')
         queryset = type(self).queryset.filter(collection__pk=collection.pk,
-                                              uid__isnull=False,
                                               revisions__current=True)
 
         return queryset