'Tag the method as a "linked to attribute" event handler.
This optional event handler will be called when the collection class
is linked to or unlinked from the InstrumentedAttribute.  It is
invoked immediately after the \'_sa_adapter\' property is set on
the instance.  A single argument is passed: the collection adapter
that has been linked, or None if unlinking.'

@staticmethod
def linker(fn):

    fn._sa_instrument_role = 'linker'
    return fn
