Question

If I perform the following query in MongoDB shell, all is fine:

db.users.find({location: {$near: [52.3, 6.6], $maxDistance: 0.27}})

However, if I switch the order of arguments, the query fails:

db.users.find({location: {$maxDistance: 0.27, $near: [52.3, 6.6]}})

Mongo returns the following error:

error: {
    "$err" : "geo values have to be numbers: { $maxDistance: 0.27, $near: [ 52.3, 6.6 ] }",
    "code" : 13026
}

Apparently, the order of the query dictionary’s arguments are important, which strikes me as odd. Can anybody shed some light on this? I’m running MongoDB 2.0.2.

The required order poses a problem for me as I build up the dictionary dynamically and (want to) have no control over the order in which the keys are serialised (which, happens to be the one posing the error).

Answer 1

Well, apparently, Mongo expects ordered dictionary (so called SONs) for some commands, like the $near-query.

In Python, this is done by casting the dictionary to a SON, like so:

from pymongo.son import SON

s = SON({'$near': [50, 50]})
s['$maxDistance'] = 0.27
db.users.find({'geoloc': s})