- :func:`.attributes.get_history()` when used with a scalar column-mapped

attribute will now honor the "passive" flag
passed to it; as this defaults to ``PASSIVE_OFF``, the function will
by default query the database if the value is not present.
This is a behavioral change vs. 0.8.  [ticket:2787]
- Added new method :meth:`.AttributeState.load_history`, works like
:attr:`.AttributeState.history` but also fires loader callables.

2 files changed, 43 insertions, 6 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index da9d62d13..e78973459 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -552,7 +552,6 @@ class AttributeImpl(object):
 
     def get(self, state, dict_, passive=PASSIVE_OFF):
         """Retrieve a value from the given object.
-
         If a callable is assembled on this object's attribute, and
         passive is False, the callable will be executed and the
         resulting value will be set as the new value for this attribute.
@@ -652,8 +651,16 @@ class ScalarAttributeImpl(AttributeImpl):
         del dict_[self.key]
 
     def get_history(self, state, dict_, passive=PASSIVE_OFF):
-        return History.from_scalar_attribute(
-            self, state, dict_.get(self.key, NO_VALUE))
+        if self.key in dict_:
+            return History.from_scalar_attribute(self, state, dict_[self.key])
+        else:
+            if passive & INIT_OK:
+                passive ^= INIT_OK
+            current = self.get(state, dict_, passive=passive)
+            if current is PASSIVE_NO_RESULT:
+                return HISTORY_BLANK
+            else:
+                return History.from_scalar_attribute(self, state, current)
 
     def set(self, state, dict_, value, initiator,
                 passive=PASSIVE_OFF, check_old=None, pop=False):
@@ -1226,7 +1233,7 @@ class History(History):
         original = state.committed_state.get(attribute.key, _NO_HISTORY)
 
         if original is _NO_HISTORY:
-            if current is NO_VALUE:
+            if current is NEVER_SET:
                 return cls((), (), ())
             else:
                 return cls((), [current], ())
@@ -1243,7 +1250,7 @@ class History(History):
                 deleted = ()
             else:
                 deleted = [original]
-            if current is NO_VALUE:
+            if current is NEVER_SET:
                 return cls((), (), deleted)
             else:
                 return cls([current], (), deleted)
diff --git a/lib/sqlalchemy/orm/state.py b/lib/sqlalchemy/orm/state.py
index 8d73c9426..957e29700 100644
--- a/lib/sqlalchemy/orm/state.py
+++ b/lib/sqlalchemy/orm/state.py
@@ -16,7 +16,7 @@ from .. import util
 from . import exc as orm_exc, interfaces
 from .path_registry import PathRegistry
 from .base import PASSIVE_NO_RESULT, SQL_OK, NEVER_SET, ATTR_WAS_SET, \
-        NO_VALUE, PASSIVE_NO_INITIALIZE
+        NO_VALUE, PASSIVE_NO_INITIALIZE, INIT_OK, PASSIVE_OFF
 from . import base
 
 class InstanceState(interfaces._InspectionAttr):
@@ -555,10 +555,40 @@ class AttributeState(object):
         """Return the current pre-flush change history for
         this attribute, via the :class:`.History` interface.
 
+        This method will **not** emit loader callables if the value of the
+        attribute is unloaded.
+
+        .. seealso::
+
+            :meth:`.AttributeState.load_history` - retrieve history
+            using loader callables if the value is not locally present.
+
+            :func:`.attributes.get_history` - underlying function
+
         """
         return self.state.get_history(self.key,
                     PASSIVE_NO_INITIALIZE)
 
+    def load_history(self):
+        """Return the current pre-flush change history for
+        this attribute, via the :class:`.History` interface.
+
+        This method **will** emit loader callables if the value of the
+        attribute is unloaded.
+
+        .. seealso::
+
+            :attr:`.AttributeState.history`
+
+            :func:`.attributes.get_history` - underlying function
+
+        .. versionadded:: 0.9.0
+
+        """
+        return self.state.get_history(self.key,
+                    PASSIVE_OFF ^ INIT_OK)
+
+
 
 class PendingCollection(object):
     """A writable placeholder for an unloaded collection.