[FIXED] correct way to nest Item data in scrapy

Issue

What is the correct way to nest Item data?

For example, I want the output of a product:

{
    'price': price,
    'title': title,
    'meta': {
        'url': url,
        'added_on': added_on
}

I have scrapy.Item of:

class ProductItem(scrapy.Item):
    url = scrapy.Field(output_processor=TakeFirst())
    price = scrapy.Field(output_processor=TakeFirst())
    title = scrapy.Field(output_processor=TakeFirst())
    url = scrapy.Field(output_processor=TakeFirst())
    added_on = scrapy.Field(output_processor=TakeFirst())

Now, the way I do it is just to reformat the whole item in the pipeline according to new item template:

class FormatedItem(scrapy.Item):
    title = scrapy.Field()
    price = scrapy.Field()
    meta = scrapy.Field()

and in pipeline:

def process_item(self, item, spider):
    formated_item = FormatedItem()
    formated_item['title'] = item['title']
    formated_item['price'] = item['price']
    formated_item['meta'] = {
        'url': item['url'],
        'added_on': item['added_on']
    }
    return formated_item

Is this correct way to approach this or is there a more straight-forward way to approach this without breaking the philosophy of the framework?

Solution

UPDATE from comments: Looks like nested loaders is the updated approach. Another comment suggests this approach will cause errors during serialization.

Best way to approach this is by creating a main and a meta item class/loader.

from scrapy.item import Item, Field
from scrapy.loader import ItemLoader
from itemloaders.processors import TakeFirst


class MetaItem(Item):
    url = Field()
    added_on = Field()


class MainItem(Item):
    price = Field()
    title = Field()
    meta = Field(serializer=MetaItem)


class MainItemLoader(ItemLoader):
    default_item_class = MainItem
    default_output_processor = TakeFirst()


class MetaItemLoader(ItemLoader):
    default_item_class = MetaItem
    default_output_processor = TakeFirst()

Sample usage:

from scrapy import Spider
from ..items import MainItemLoader, MetaItemLoader
from scrapy.selector import Selector


class DmozSpider(Spider):
    name = "dmoz"
    allowed_domains = ["example.com"]
    start_urls = ["http://example.com"]

    def parse(self, response):
        mainloader = MainItemLoader(selector=Selector(response))
        mainloader.add_value('title', 'test')
        mainloader.add_value('price', 'price')
        mainloader.add_value('meta', self.get_meta(response))
        return mainloader.load_item()

    def get_meta(self, response):
        metaloader = MetaItemLoader(selector=Selector(response))
        metaloader.add_value('url', response.url)
        metaloader.add_value('added_on', 'now')
        return metaloader.load_item()

After that, you can easily expand your items in the future by creating more "sub-items."

Answered By - marven