Add 'can_be_moved' method in 'PolymorphicMPTTModel'

Author: Anton Prokhorov

polymorphic_tree/models.py

@@ -111,6 +111,20 @@ def get_ancestors_of_type(self, model, ascending=False, include_self=False):
  """
  return self.get_ancestors(ascending=ascending, include_self=include_self).instance_of(model)
 
+ def can_be_moved(self, target):
+ """Can move be finished
+
+ Method have to be redefined in inherited model to define cases when
+ node can be moved. If method is not redefined moving always allows
+
+ To deny move, this method have to be raised ``ValidationError`` or
+ ``InvalidMove`` from ``mptt.exceptions``
+
+ Args:
+ target (PolymorphicMPTTModel): future parent of node
+ """
+ return True
+
 
 if django.VERSION < (1,7):
  # South integration