python - mongodb的cursor.count()是实际计数吗？

Question

当我扫描 mongodb 中的集合并编辑所有项目时，我得到了这个问题:扫描计数器 i 可以大于cursor.count()。 为什么会发生这种情况？有人能猜出来吗？

db.baiduwords.find().count() = 805280

db.baiduwords.find().size() = 805280

但是我达到了 1498400 或更多。

import pymongo
if __name__=='__main__':
    client = pymongo.MongoClient()
    i = 0
    collection = client.baike.baiduwords.find()
    for item in collection:
        i += 1
        MajorClass = []
        for cl in item['C']:
            c = cl
            while(1):
                a = client.baike.baiduclass.find({'s':c})
                if a.count():
                    a = a[0]
                    if a['f'] == u'Root':
                        MajorClass.append(c)
                        break
                    else:
                        c = a['f']
                else:
                    break
        item['MC'] = list(set(MajorClass))
        client.baike.baiduwords.save(item)
        if i%100 == 0:
            print "%d/%d"%(i, collection.count())

PS:printShardingStatus:此数据库未启用分片。

Answer 1

当您保存项目时，更新的文档可能会移动，导致光标多次返回该文档。

来自http://docs.mongodb.org/manual/faq/developers/#how-do-i-isolate-cursors-from-intervening-write-operations :

As a cursor returns documents, other operations may interleave with the query: if some of these operations are updates that cause the document to move (in the case of a table scan, caused by document growth,) or that change the indexed field on the index used by the query; then the cursor will return the same document more than once.