Querying a list in mongoengine; contains vs in

问题

I have a ListField in a model with ids (ReferenceField), and I need to do a query if a certain id is in that list. AFAIK I have 2 options for this:

Model.objects.filter(refs__contains='59633cad9d4bc6543aab2f39')

or:

Model.objects.filter(refs__in=['59633cad9d4bc6543aab2f39'])

Which one is the most efficient for this use case?

The model looks like:

class Model(mongoengine.Document):
    refs = mongoengine.ListField(mongoengine.ReferenceField(SomeOtherModel))

From what I can read in the mongoengine documentation, http://docs.mongoengine.org/guide/querying.html#string-queries, contains is really a string query, but it works surprisingly here as well. But I'm guessing that __in is more efficient since it should be optimized for lists, or am I wrong?

回答1:

The string queries normally under the covers are all regex query so would be less efficient. However, the exception is when testing against reference fields! The following queries are:

Model.objects.filter(refs__contains="5305c92956c02c3f391fcaba")._query
{'refs': ObjectId('5305c92956c02c3f391fcaba')}

Which is a direct lookup.

Model.objects.filter(refs__in=["5305c92956c02c3f391fcaba"])._query
{'refs': {'$in': [ObjectId('5305c92956c02c3f391fcaba')]}}

This probably is less efficient, but would probably be extremely marginal. The biggest impact would be the number of docs and whether or not the refs field has an index.

来源：https://stackoverflow.com/questions/21888096/querying-a-list-in-mongoengine-contains-vs-in