mongodb $exists always returning 0

问题

I have a database collection (named fols) like so:

{'followers':
        {
           '123':1
           '123':2
           '123':3
         }
}

If I run the query (Using pymongo):

cursor = fols.find()
cursor.count()
>>3

Works fine. Now:

cursor = fols.find({'followers':{'123':1}})
cursor.count()
>>1

Again works fine. BUT if I try:

cursor = fols.find({'followers':{'123':{'$exists': True}}})
cursor.count()
>> 0

It returns 0 even though there are 3 records.

回答1:

When you're not matching a complete object you need to use dot notation to use an operator against an embedded object. So in this case:

cursor = fols.find({'followers.123':{'$exists': True}})

回答2:

Try the dot syntax:

cursor = fols.find({'followers.123': {'$exists': True}})

But also see my comment above. You can't have the same key more than once in a (sub-)document.

来源：https://stackoverflow.com/questions/12398625/mongodb-exists-always-returning-0