diff --git a/src/grpc_client/models_accessor.py b/src/grpc_client/models_accessor.py
index 29c2f11..188a9ce 100644
--- a/src/grpc_client/models_accessor.py
+++ b/src/grpc_client/models_accessor.py
@@ -11,14 +11,13 @@
         Give a Model Class Name and some data, check if that exits or instantiate a new one
         """
 
-
         if data.get('name'):
             used_key = 'name'
         else:
             used_key = data.keys()[0]
 
         if class_name not in RESOURCES:
-            raise Exception('[XOS-TOSCA] The model your tring to create (%s: %s, class: %s) is not know by xos-core' % (used_key, data[used_key], class_name))
+            raise Exception('[XOS-TOSCA] The model you are trying to create (%s: %s, class: %s) is not know by xos-core' % (used_key, data[used_key], class_name))
 
         cls = RESOURCES[class_name]
         models = cls.objects.filter(**{used_key: data[used_key]})
@@ -27,6 +26,10 @@
             print "[XOS-Tosca] Model %s already exist, retrieving instance..." % data[used_key]
             model = models[0]
         elif len(models) == 0:
+
+            if 'must-exist' in data and data['must-exist']:
+                raise Exception("[XOS-TOSCA] Model %s:%s has property 'must-exist' but cannot be found" % (class_name, data[used_key]))
+
             model = cls.objects.new()
             print "[XOS-Tosca] Model %s is new, creating new instance..." % data[used_key]
         else: