当我试图用scrapy刮刮Google App Store时，我无法将数据保存到MongoDB。我收到一个错误：bson.errors.InvalidDocument: cannot encode object。我在Google上搜索了此错误，结果表明数据类型存在问题，但是我已经将其转换为字典数据类型。

我的Pipeline.py文件是：

from pymongo import MongoClient
from scrapy import Item


class GpPipeline(object):
    def open_spider(self,spider):
        db_uri = spider.settings.get('MONGODB_URI','mongodb://localhost:27017')
        db_name = spider.settings.get('MONGODB_DBNAME','scrapy_db')
        db_post = spider.settings.get('MONGODB_DOCNAME','apkinfo')

        self.db_client = MongoClient(db_uri)
        self.db = self.db_client[db_name]
        self.post = self.db[db_post]

    def close_spider(self,spider):
        self.db_client.close()

    def process_item(self,item,spider):
        postItem = dict(item)
        self.post.insert(postItem)
        return item

    def insert_db(self,item):
        if isinstance(item,Item):
            item = dict(item)
        self.db.books.insert(item)

来自终端的错误信息基本上如下：

2019-11-17 00:50:18 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://play.google.com/store/apps/details?id=com.pinterest> (referer: None)
2019-11-17 00:50:18 [scrapy.core.scraper] ERROR: Error processing {'app_icon': [<Selector xpath='//img[@itemprop="image"]/@src' data='https://lh3.googleusercontent.com/dVsv8H'>,<Selector xpath='//img[@itemprop="image"]/@src' data='https://lh3.googleusercontent.com/HotsP0'>,<Selector xpath='//img[@itemprop="image"]/@src' data='https://lh3.googleusercontent.com/jLIpel'>,<Selector xpath='//img[@itemprop="image"]/@src' data='https://lh3.googleusercontent.com/4CInZm'>],'app_name': [],'app_url': 'https://play.google.com/store/apps/details?id=com.pinterest'}
Traceback (most recent call last):
  File "/anaconda3/lib/python3.7/site-packages/twisted/internet/defer.py",line 649,in _runCallbacks
    current.result = callback(current.result,*args,**kw)
  File "/Users/zhanjinyang/Desktop/gp/gp/pipelines.py",line 27,in process_item
    self.post.insert(postItem)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/collection.py",line 3195,in insert
    check_keys,manipulate,write_concern)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/collection.py",line 614,in _insert
    bypass_doc_val,session)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/collection.py",line 602,in _insert_one
    acknowledged,_insert_command,session)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/mongo_client.py",line 1280,in _retryable_write
    return self._retry_with_session(retryable,func,s,None)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/mongo_client.py",line 1233,in _retry_with_session
    return func(session,sock_info,retryable)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/collection.py",line 597,in _insert_command
    retryable_write=retryable_write)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/pool.py",line 589,in command
    self._raise_connection_failure(error)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/pool.py",line 750,in _raise_connection_failure
    raise error
  File "/anaconda3/lib/python3.7/site-packages/pymongo/pool.py",line 584,in command
    user_fields=user_fields)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/network.py",line 121,in command
    codec_options,ctx=compression_ctx)
  File "/anaconda3/lib/python3.7/site-packages/pymongo/message.py",line 678,in _op_msg
    flags,command,identifier,docs,check_keys,opts)
bson.errors.InvalidDocument: cannot encode object: <Selector xpath='//img[@itemprop="image"]/@src' data='https://lh3.googleusercontent.com/dVsv8H'>,of type: <class 'scrapy.selector.unified.Selector'>