'Add an individual MapperProperty to this mapper.
If the mapper has not been configured yet, just adds the
property to the initial properties dictionary sent to the
constructor.  If this Mapper has already been configured, then
the given MapperProperty is configured immediately.'

def add_property(self, key, prop):

    self._init_properties[key] = prop
    self._configure_property(key, prop, init=self.configured)
