diff -r 906a6c7c7943 -r 6f3078f7fd47 src/notes/api/serializers/core.py
--- a/src/notes/api/serializers/core.py	Tue Aug 01 12:20:14 2017 +0200
+++ b/src/notes/api/serializers/core.py	Thu Aug 03 09:44:37 2017 +0200
@@ -6,6 +6,7 @@
 from django.contrib.auth.models import Group
 from rest_framework import serializers
 
+from notes.api.fields.category import ProtocolField
 from notes.models import Note, Session
 
 logger = logging.getLogger(__name__)
@@ -90,6 +91,7 @@
     owner = serializers.SlugRelatedField(
         read_only=True, slug_field='username', default=serializers.CurrentUserDefault())
     group = serializers.SlugRelatedField(read_only=True, slug_field='name')
+    protocol = ProtocolField(required=False)
 
     class Meta:
         model = Session
@@ -97,7 +99,7 @@
             'ext_id', 'version', 'date', 'created', 'updated',
             'owner', 'title', 'description', 'protocol', 'group'
         )
-        read_only_fields = ('ext_id', 'version', 'created', 'updated', 'owner', 'group')
+        read_only_fields = ('ext_id', 'version', 'created', 'updated', 'owner', 'group', 'protocol')
 
 
 class DetailSessionSerializer(serializers.ModelSerializer):
@@ -105,6 +107,7 @@
     owner = serializers.SlugRelatedField(read_only=True, slug_field='username')
     notes = DetailNoteSerializer(many=True, read_only=True)
     group = serializers.SlugRelatedField(slug_field='name', read_only=True)
+    protocol = ProtocolField(required=False)
 
     class Meta:
         model = Session
@@ -113,13 +116,14 @@
             'owner', 'title', 'description', 'protocol', 'group',
             'notes'
         )
-        read_only_fields = ('ext_id', 'version', 'created', 'updated', 'owner', 'group')
+        read_only_fields = ('ext_id', 'version', 'created', 'updated', 'owner', 'group', 'protocol')
 
 class CreateSessionSerializer(serializers.ModelSerializer):
 
     owner = serializers.SlugRelatedField(
         read_only=True, slug_field='username', default=serializers.CurrentUserDefault())
-    group = serializers.SlugRelatedField(slug_field='name', queryset=Group.objects.all(), required=False)
+    group = serializers.SlugRelatedField(slug_field='name', queryset=Group.objects.all(), required=False, allow_null=True)
+    protocol = ProtocolField(required=False)
 
     class Meta:
         model = Session
@@ -148,4 +152,3 @@
 
         data['group'] = group
         return data
-