[SQLAlchemy] synchronize_session參數詳解

synchronize_session用於query在進行delete or update操作時，對session的同步策略。

• False - 不對session進行同步，直接進行delete or update操作。
• 'fetch'

　　　在delete or update操作之前，先發一條sql到數據庫獲取符合條件的記錄。

def _do_pre_synchronize(self):
        query = self.query
        session = query.session
        context = query._compile_context()
        select_stmt = context.statement.with_only_columns(
            self.primary_table.primary_key)
        self.matched_rows = session.execute(
            select_stmt,
            mapper=self.mapper,
            params=query._params).fetchall()

　　在delete or update操作之后，將session的identity_map與前一步獲取到的記錄進行match，符合條件的就從session中刪掉或更新。

def _do_post_synchronize(self):
        session = self.query.session
        target_mapper = self.query._mapper_zero()
        for primary_key in self.matched_rows:
            # TODO: inline this and call remove_newly_deleted
            # once
            identity_key = target_mapper.identity_key_from_primary_key(
                list(primary_key))
            if identity_key in session.identity_map:
                session._remove_newly_deleted(
                    [attributes.instance_state(
                        session.identity_map[identity_key]
                    )]
                )

• 'evaluate'

　　　在delete or update操作之前，用query中的條件直接對session的identity_map中的objects進行eval操作，將符合條件的記錄下來。

def _do_pre_synchronize(self):
        query = self.query
        target_cls = query._mapper_zero().class_

        try:
            evaluator_compiler = evaluator.EvaluatorCompiler(target_cls)
            if query.whereclause is not None:
                eval_condition = evaluator_compiler.process(
                    query.whereclause)
            else:
                def eval_condition(obj):
                    return True

            self._additional_evaluators(evaluator_compiler)

        except evaluator.UnevaluatableError:
            raise sa_exc.InvalidRequestError(
                "Could not evaluate current criteria in Python. "
                "Specify 'fetch' or False for the "
                "synchronize_session parameter.")

        # TODO: detect when the where clause is a trivial primary key match
        self.matched_objects = [
            obj for (cls, pk), obj in
            query.session.identity_map.items()
            if issubclass(cls, target_cls) and
            eval_condition(obj)]

　　　在delete or update操作之后，將符合條件的記錄刪除或更新。

def _do_post_synchronize(self):
        self.query.session._remove_newly_deleted(
            [attributes.instance_state(obj)
             for obj in self.matched_objects])