python - pymongo排序返回没有排序键的记录

Question

我正在对集合的日期列进行排序，以查找该集合中的最小日期。但是，它返回给我一条缺少日期键的记录。下面是片段。这是一个错误吗？

date_records = usercollection.find({'customer_id':'abc'}).sort('join_date',1).limit(1)
for record in date_records:
    print record # prints a record that doesn't have the join_date key
    print record['join_date']

输出:

{ "_id" : ObjectId("94dbe4c6ea890e28113d7664"), "region" : "Virginia", "country_code" : "US", "customer_id" : "abc"}

KeyError: u'join_date'

Answer 1

这不是一个错误以及如何sort in MongoDB应该可以工作:

The comparison treats a non-existent field as it would an empty BSON Object. As such, a sort on the a field in documents { } and { a: null } would treat the documents as equivalent in sort order.

而且，既然您已经注意到:

I want to retrieve the earliest date from all records that have the join_date field

使用$exists检查join_date是否存在:

usercollection.find({
    'customer_id': 'abc',
    'join_date': {'$exists': True}
}).sort('join_date', 1)