- The ``viewonly`` flag on :func:`.relationship` will now prevent

attribute history from being written on behalf of the target attribute.
This has the effect of the object not being written to the
Session.dirty list if it is mutated.  Previously, the object would
be present in Session.dirty, but no change would take place on behalf
of the modified attribute during flush.   The attribute still emits
events such as backref events and user-defined events and will still
receive mutations from backrefs. [ticket:2833]

3 files changed, 9 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index e78973459..6071b565d 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -370,6 +370,7 @@ class AttributeImpl(object):
                     callable_, dispatch, trackparent=False, extension=None,
                     compare_function=None, active_history=False,
                     parent_token=None, expire_missing=True,
+                    send_modified_events=True,
                     **kwargs):
         """Construct an AttributeImpl.
 
@@ -413,6 +414,10 @@ class AttributeImpl(object):
           during state.expire_attributes(None), if no value is present
           for this key.
 
+        send_modified_events
+          if False, the InstanceState._modified_event method will have no effect;
+          this means the attribute will never show up as changed in a
+          history entry.
         """
         self.class_ = class_
         self.key = key
@@ -420,6 +425,7 @@ class AttributeImpl(object):
         self.dispatch = dispatch
         self.trackparent = trackparent
         self.parent_token = parent_token or self
+        self.send_modified_events = send_modified_events
         if compare_function is None:
             self.is_equal = operator.eq
         else:
diff --git a/lib/sqlalchemy/orm/state.py b/lib/sqlalchemy/orm/state.py
index 957e29700..ddd991acc 100644
--- a/lib/sqlalchemy/orm/state.py
+++ b/lib/sqlalchemy/orm/state.py
@@ -415,6 +415,8 @@ class InstanceState(interfaces._InspectionAttr):
         return None
 
     def _modified_event(self, dict_, attr, previous, collection=False):
+        if not attr.send_modified_events:
+            return
         if attr.key not in self.committed_state:
             if collection:
                 if previous is NEVER_SET:
diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 009bf74a4..b04338d9c 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -80,6 +80,7 @@ def _register_attribute(strategy, mapper, useobject,
                 callable_=callable_,
                 active_history=active_history,
                 impl_class=impl_class,
+                send_modified_events=not useobject or not prop.viewonly,
                 doc=prop.doc,
                 **kw
                 )