Method _synchronize

lib/sqlalchemy/orm/dependency.py:829–867

(
        self,
        state,
        child,
        associationrow,
        clearkeys,
        uowcommit,
        operation=None,
    )

                    self._post_update(state, uowcommit, history.sum())

    def _synchronize(
        self,
        state,
        child,
        associationrow,
        clearkeys,
        uowcommit,
        operation=None,
    ):
        if state is None or (
            not self.post_update and uowcommit.is_deleted(state)
        ):
            return

        if (
            operation is not None
            and child is not None
            and not uowcommit.session._contains_state(child)
        ):
            util.warn(
                "Object of type %s not in session, %s "
                "operation along '%s' won't proceed"
                % (mapperutil.state_class_str(child), operation, self.prop)
            )
            return

        if clearkeys or child is None:
            sync._clear(state, self.parent, self.prop.synchronize_pairs)
        else:
            self._verify_canload(child)
            sync._populate(
                child,
                self.mapper,
                state,
                self.parent,
                self.prop.synchronize_pairs,
                uowcommit,
                False,
            )


class _DetectKeySwitch(_DependencyProcessor):