如何在scrapy中添加一个值来描述值来自哪个链接

import scrapy

class MyItem(scrapy.Item):
    # 其他字段...
    source_link = scrapy.Field()

import scrapy
from scrapy.loader import ItemLoader
from myproject.items import MyItem

class MySpider(scrapy.Spider):
    name = 'my_spider'

    def start_requests(self):
        # 起始链接...
        yield scrapy.Request(url='http://example.com', callback=self.parse)

    def parse(self, response):
        # 解析页面...

        # 使用ItemLoader赋值
        loader = ItemLoader(item=MyItem(), response=response)
        loader.add_value('source_link', response.url)
        item = loader.load_item()

        # 或直接赋值
        item = MyItem()
        item['source_link'] = response.url

        yield item