json.py 文件源码

def setup_default_value_handling(cls):
    """A SQLAlchemy model class decorator that ensures JSON/JSONB default values are correctly handled.

    Settings default values for JSON fields have a bunch of issues, because dict and list instances need to be wrapped in ``NestedMutationDict`` and ``NestedMutationList`` that correclty set the parent object dirty state if the container content is modified. This class decorator sets up hooks, so that default values are automatically converted to their wrapped versions.

    .. note ::

        We are only concerned about initial values. When values are loaded from the database, ``Mutable`` base class of ``NestedMutationDict`` correctly sets up the parent pointers.

    .. note ::

        This must be applid to the class directly, as SQLAlchemy does not seem ot pick it up if it's applied to an (abstract) parent class.

    This might be addressed in future SQLAlchemy / Websauna versions so that we can get rid of this ugly little decorator.
    """

    @event.listens_for(cls, "init")
    def init(target, args, kwargs):
        for c in target.__table__.columns:
            if is_json_like_column(c):
                default =_get_column_default(target, c)
                default = wrap_as_nested(c.name, default, target)
                setattr(target, c.name, default)

    return cls