renaming the value manager's initial_value field to previous_value

cqlengine/columns.py

@@ -12,12 +12,22 @@ class BaseValueManager(object):
     def __init__(self, instance, column, value):
         self.instance = instance
         self.column = column
-        self.initial_value = copy(value)
+        self.previous_value = copy(value)
         self.value = value
 
     @property
     def deleted(self):
-        return self.value is None and self.initial_value is not None
+        return self.value is None and self.previous_value is not None
+
+    @property
+    def changed(self):
+        """
+        Indicates whether or not this value has changed.
+
+        :rtype: boolean
+
+        """
+        return self.value != self.previous_value
 
     def getval(self):
         return self.value

cqlengine/tests/model/test_model_io.py

@@ -62,7 +62,7 @@ class TestModelIO(BaseCassEngTestCase):
 
         tm2 = TestModel.objects(id=tm.pk).first()
         assert tm2.text is None
-        assert tm2._values['text'].initial_value is None
+        assert tm2._values['text'].previous_value is None
 
 
     def test_a_sensical_error_is_raised_if_you_try_to_create_a_table_twice(self):