Fix unpickling pre-3.28.0 Transformation

At around 3.28 we introduced domain caching into Tranformation in a
way that prevented older pickles to work. This fix recreates the cache
after unpickling. Furthermore, the cache (before "need_domain", now
"_target_domain") is never pickled anymore.

Orange/preprocess/transformation.py

@@ -16,12 +16,28 @@ def __init__(self, variable):
         :type variable: int or str or :obj:`~Orange.data.Variable`
         """
         self.variable = variable
+        self._create_cached_target_domain()
 
+    def _create_cached_target_domain(self):
+        """ If the same domain is used everytime this allows better caching of
+        domain transformations in from_table"""
         if self.variable is not None:
             if self.variable.is_primitive():
-                self.need_domain = Domain([self.variable])
+                self._target_domain = Domain([self.variable])
             else:
-                self.need_domain = Domain([], metas=[self.variable])
+                self._target_domain = Domain([], metas=[self.variable])
+
+    def __getstate__(self):
+        # Do not pickle the cached domain; rather recreate it after unpickling
+        state = self.__dict__.copy()
+        state.pop("_target_domain")
+        return state
+
+    def __setstate__(self, state):
+        # Ensure that cached target domain is created after unpickling.
+        # This solves the problem of unpickling old pickled models.
+        self.__dict__.update(state)
+        self._create_cached_target_domain()
 
     def __call__(self, data):
         """
@@ -31,7 +47,7 @@ def __call__(self, data):
         inst = isinstance(data, Instance)
         if inst:
             data = Table.from_list(data.domain, [data])
-        data = data.transform(self.need_domain)
+        data = data.transform(self._target_domain)
         if self.variable.is_primitive():
             col = data.X
         else:

Orange/tests/test_transformation.py

@@ -1,3 +1,4 @@
+import pickle
 import unittest
 
 import numpy as np
@@ -43,6 +44,17 @@ def test_transform_fails(self):
         trans = Transformation(self.data.domain[2])
         self.assertRaises(NotImplementedError, trans, self.data)
 
+    def test_pickling_target_domain(self):
+        data = self.data
+        trans = self.TransformationMock(data.domain[2])
+        self.assertIn("_target_domain", trans.__dict__)
+        # _target_domain should not be pickled
+        state = trans.__getstate__()
+        self.assertNotIn("_target_domain", state)
+        # _target_domain should be recreated when unpickled
+        unpickled = pickle.loads(pickle.dumps(trans))
+        self.assertIn("_target_domain", unpickled.__dict__)
+
 
 class IdentityTest(unittest.TestCase):
     def test_identity(self):