How to query through a DBRef in MongoDB/pymongo?

Question

Is it possible to query through a DBRef using a single find spec?

user collection

{
    'age': 30
}

post collection

{
    'user': DBRef('user', ...)
}

Is it possible to query for all post who's users are 30 in a single find step? If not, would it be wise to create a javascript function to handle the multi-stage operation or will that cause blocking problems?

score 6 · Accepted Answer · answered May 19 '10 at 17:57

6

it's not possible to do that. i would recommend either:

a) changing your data model so that all of the data is in a single document (might not be possible depending on your case).

b) querying for users who are 30 first, and then doing a second query to get posts where user is $in that list. i would do this client side rather than using server-side JS or anything like that.

answered May 19 '10 at 17:57

mdirolf

7,521
2
23
15

A single document isn't possible at this point and I'm already going with option B. Thanks. – Soviut May 19 '10 at 18:51
I just had this problem too, and I think map/reduce may be an option too. http://www.mongodb.org/display/DOCS/MapReduce – sandstrom Sep 16 '10 at 19:42
how to compose the $in query? could you please come up with an example? – Gelin Luo Jun 01 '11 at 01:20
@green I've added an example in the answer below – MFB Oct 11 '12 at 04:57

score 1 · Answer 2 · answered Oct 11 '12 at 04:57

1

I use a Python driver, so forgive my not-so-mongodb syntax:

users = list(db.Users.find({'Age':30}))
posts = list(db.Posts.find({'User':{'$in':users}}))

answered Oct 11 '12 at 04:57

MFB

19,017
27
72
118

score 0 · Answer 3 · answered Apr 16 '14 at 07:43

Install python bson package. and try as example.

import pymongo
from pymongo import MongoClient
from bson.dbref import DBRef

client = MongoClient('ip', 27017)

client.the_database.authenticate('user', 'password', source='db_name')
db = client['db_name']
user = db['user']
user_id = user.find_one({'email': 'xxxxx@gmail.com'}).get('_id')

client_user_relation = db['client_user_relation']
print(client_user_relation.find_one())
print(user_id)
print(DBRef(collection = "user", id = user_id))
print(client_user_relation.find_one({'user': DBRef(collection = "user", id = user_id)}))

How to query through a DBRef in MongoDB/pymongo?

3 Answers3