Method before_update

lib/sqlalchemy/orm/events.py:1272–1343

Receive an object instance before an UPDATE statement is emitted corresponding to that instance. .. note:: this event **only** applies to the :ref:`session flush operation <session_flushing>` and does **not** apply to the ORM DML operations described at

(
        self, mapper: Mapper[_O], connection: Connection, target: _O
    )

        """

    def before_update(
        self, mapper: Mapper[_O], connection: Connection, target: _O
    ) -> None:
        """Receive an object instance before an UPDATE statement
        is emitted corresponding to that instance.

        .. note:: this event **only** applies to the
           :ref:`session flush operation <session_flushing>`
           and does **not** apply to the ORM DML operations described at
           :ref:`orm_expression_update_delete`.  To intercept ORM
           DML events, use :meth:`_orm.SessionEvents.do_orm_execute`.

        This event is used to modify local, non-object related
        attributes on the instance before an UPDATE occurs, as well
        as to emit additional SQL statements on the given
        connection.

        This method is called for all instances that are
        marked as "dirty", *even those which have no net changes
        to their column-based attributes*. An object is marked
        as dirty when any of its column-based attributes have a
        "set attribute" operation called or when any of its
        collections are modified. If, at update time, no
        column-based attributes have any net changes, no UPDATE
        statement will be issued. This means that an instance
        being sent to :meth:`~.MapperEvents.before_update` is
        *not* a guarantee that an UPDATE statement will be
        issued, although you can affect the outcome here by
        modifying attributes so that a net change in value does
        exist.

        To detect if the column-based attributes on the object have net
        changes, and will therefore generate an UPDATE statement, use
        ``object_session(instance).is_modified(instance,
        include_collections=False)``.

        The event is often called for a batch of objects of the
        same class before their UPDATE statements are emitted at
        once in a later step. In the extremely rare case that
        this is not desirable, the :class:`_orm.Mapper` can be
        configured with ``batch=False``, which will cause
        batches of instances to be broken up into individual
        (and more poorly performing) event->persist->event
        steps.

        .. warning::

            Mapper-level flush events only allow **very limited operations**,
            on attributes local to the row being operated upon only,
            as well as allowing any SQL to be emitted on the given
            :class:`_engine.Connection`.  **Please read fully** the notes
            at :ref:`session_persistence_mapper` for guidelines on using
            these methods; generally, the :meth:`.SessionEvents.before_flush`
            method should be preferred for general on-flush changes.

        :param mapper: the :class:`_orm.Mapper` which is the target
         of this event.
        :param connection: the :class:`_engine.Connection` being used to