Document and support nested composites

Composites can behave in a "nested" fashion by defining the
class in that way.   To make the constructor more convenient,
a callable can be passed to :func:`.composite` instead of the
class itself.  This works now, so add a test to ensure this
pattern remains available.

Change-Id: Ia009f274fca7269f41d6d824e0f70b6fb0ada081

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/descriptor_props.py b/lib/sqlalchemy/orm/descriptor_props.py
index aaf53b698..3fefc5bba 100644
--- a/lib/sqlalchemy/orm/descriptor_props.py
+++ b/lib/sqlalchemy/orm/descriptor_props.py
@@ -100,7 +100,9 @@ class CompositeProperty(DescriptorProperty):
         is the :class:`.CompositeProperty`.
 
         :param class\_:
-          The "composite type" class.
+          The "composite type" class, or any classmethod or callable which
+          will produce a new instance of the composite object given the
+          column values in order.
 
         :param \*cols:
           List of Column objects to be mapped.