the consideration of a pending object as

the consideration of a pending object as
an "orphan" has been modified to more closely match the
behavior as that of persistent objects, which is that the object
is expunged from the :class:`.Session` as soon as it is
de-associated from any of its orphan-enabled parents.  Previously,
the pending object would be expunged only if de-associated
from all of its orphan-enabled parents.  The new flag ``legacy_is_orphan``
is added to :func:`.orm.mapper` which re-establishes the
legacy behavior. [ticket:2655]