Mongodb with Python's "set()" type

Question

I am building a web App with mongoDB as the backend. Some of the documents need to store a collection of items in some sort of list, and then the system will need to frequently check if a specified item is present in that list. Using Python's 'in' operator takes Big-O(N) time, n being the size of the list. Since these list can get quite large, I want something faster than that. Python's 'set' type does this operation in constant time (and enforces uniqueness, which is good in my case), but is considered an invalid data type to put in MongoDB.

So what's the best way to do this? Is there some way to just use a regular list and exploit mongo's indexing features? Again, I want to know, for a given document in a collection, does a list inside that document contain particular element?

Answer 1

You can represent a set using a dictionary. Your elements become the keys, and all the values can be set to a constant such as 1. The in operator checks for the existence of a key.

EDIT. MongoDB stores a dict as a BSON document, where the keys must be strings (with some additional restrictions), so the above advice is of limited use.