customize relationfield serializer to bypass empty relations

src/iosanita/contenttypes/restapi/serializers/configure.zcml

@@ -13,4 +13,7 @@
       factory=".summary.JSONSummarySerializerMetadata"
       name="iosanita.contenttypes.summary_serializer_metadata"
       />
+
+  <adapter factory=".relationfield.RelationListFieldSerializer" />
+
 </configure>

src/iosanita/contenttypes/restapi/serializers/relationfield.py

@@ -0,0 +1,21 @@
+from plone.dexterity.interfaces import IDexterityContent
+from plone.restapi.interfaces import IFieldSerializer
+from z3c.relationfield.interfaces import IRelationList
+from zope.component import adapter
+from zope.interface import implementer
+from plone.restapi.serializer.relationfield import (
+    RelationListFieldSerializer as BaseSerializer,
+)
+from iosanita.contenttypes.interfaces import IIosanitaContenttypesLayer
+
+
+@adapter(IRelationList, IDexterityContent, IIosanitaContenttypesLayer)
+@implementer(IFieldSerializer)
+class RelationListFieldSerializer(BaseSerializer):
+
+    def __call__(self):
+        """
+        Do not return broken relations
+        """
+        data = super().__call__()
+        return [x for x in data if x is not None]