.. change::
    :tags: bug, orm
    :tickets: 9519

    Fixed regression where accessing the expression value of a hybrid property
    on a class that was either unmapped or not-yet-mapped (such as calling upon
    it within a :func:`_orm.declared_attr` method) would raise an internal
    error, as an internal fetch for the parent class' mapper would fail and an
    instruction for this failure to be ignored were inadvertently removed in
    2.0.
