mongo:the return don't equal count()

Question

I have a problem that's been troubling me for a long time and it follows (code demo for example):

from mongoengine import*

Class Scan(Documnet):

      name=StringField()
      .....

queryset=Scan.objects.filter(name="Bob")

number1=queryset.count()

number2=len(queryset)

However，number1=1782 and number2=1668, number1 != number2

Anyone can tell me the reason?

I would use DB queries logging (http://stackoverflow.com/questions/4375784/log-all-sql-queries) to figure out the difference between two methods and see where it all goes wrong. — wanaryytel, Sep 26 '16 at 09:30
thanks for you sharing ,but I am still in trouble.I can not catch that — DlutAF, Sep 26 '16 at 09:54
@Styvane yep，I deploy mongo server in 4 service。is it data consistency problem？ — DlutAF, Sep 26 '16 at 12:10
@Styvane so…what should I learn about to solve my problem？ — DlutAF, Sep 26 '16 at 12:24

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

This is is due to one of the following reasons as mentioned in the documentation.

On a sharded cluster, db.collection.count() can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.

To avoid these situations, on a sharded cluster, use the $group stage of the db.collection.aggregate() method to $sum the documents. For example, the following operation counts the documents in a collection:

You can use the aggregate method to do this as suggested in the documentation.

Scan.aggregate(
    {'$group': {
        '_id': None, 
        'count': {'$sum': 1}
    }}
)

You did me a big favor！ Thks!! – DlutAF Sep 26 '16 at 12:33 — DlutAF, Sep 26 '16 at 12:33

mongo:the return don't equal count()

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