support lambdas / strings in column_property

[Mike Bayer]

mostly to support mixins, e.g.

class mymixin(object):
    bar = Column(Integer)
    foo = Column(Integer)

    @declared_attr
    def foo_plus_bar(cls):
        return column_property(lambda: cls.foo + cls.bar)

[Mike Bayer]

this also needs docs. the docs at http://docs.sqlalchemy.org/en/rel_0_9/orm/extensions/declarative.html#mixing-in-deferred-column-property-and-other-mapperproperty-classes are totally useless here - the lambda: approach is needed for any column_property used in a mixin since the real columns are not available yet.

[Mike Bayer]

also, duh, we need this, this can likely be in 0.9:

diff --git a/lib/sqlalchemy/orm/properties.py b/lib/sqlalchemy/orm/properties.py
index a0def7d..26196c1 100644
--- a/lib/sqlalchemy/orm/properties.py
+++ b/lib/sqlalchemy/orm/properties.py
@@ -148,6 +148,9 @@ class ColumnProperty(StrategizedProperty):
                                     ("instrument", self.instrument)
                                     )

+    def __clause_element__(self):
+        return self.expression
+
     @property
     def expression(self):
         """Return the primary column or expression for this ColumnProperty.

[Mike Bayer]

just created #3066 for that second part

[Mike Bayer]

Duplicate of #3150.