Question

I am using Pymongo to access Mongo db. I want to search for all people nearby a specified location with name contains a string. For example, I want to search all people nearby [105.0133, 21.3434] and name contains ‘Mark’. So I write the query like this:

db.users.find({ "location.coords": { "$nearSphere": [105.0133, 21.3434], "$maxDistance": 10/EARTH_RADIUS }, "name": "/Mark/" })

(I have an index “location.coords” in my “users” collection)

The query works fine in Mongodb console, but while execute by Pymongo, the dictionary being re-sort like this:

{ "name": "/Mark/", "location.coords": { "$nearSphere": [105.0133, 21.3434], "$maxDistance": 10/EARTH_RADIUS } }

(The “name” key is before “location.coords”, that is not what I expected – also Mongodb expected)

That causes Mongodb cannot understand the query and returns no results. Can anyone help me to figure out how to force the Pymongo does not re-sort my dictionary.

Thanks and regards

Answer 1

The dictionary type is inherently orderless. From the python documentation:

It is best to think of a dictionary as an unordered set of key: value
pairs, with the requirement that the keys are unique (within one
dictionary).

If you want to index your dictionary in a specific order, you’ll have to store your order somehow. One easy way to do this is to keep your keys in a list, like:

mongo_keys = ["location.coords", "name"]
for k in mongo_keys:
    do_something(mongo_result[k])

You also might want to investigate:

class collections.OrderedDict([items])

Return an instance of a dict
subclass, supporting the usual dict methods. An OrderedDict is a dict
that remembers the order that keys were first inserted. If a new entry
overwrites an existing entry, the original insertion position is left
unchanged. Deleting an entry and reinserting it will move it to the
end.

Unfortunately if you need more help than that, you’ll need to provide more details of your situation.