MongoDB return True if document exists

I want to return true if a userID already exists and false otherwise from my collection.I have this function but it always returns True.

def alreadyExists(newID):
    if db.mycollection.find({'UserIDS': { "$in": newID}}):
        return True
    else:
        return False

How could I get this function to only return true if a user id already exists?

Note: This answer is outdated. More recent versions of MongoDB can use the far more efficient method db.collection.countDocuments. See the answer by Xavier Guihot for a better solution.

find doesn't return a boolean value, it returns a cursor. To check if that cursor contains any documents, use the cursor's count method:

if db.mycollection.find({'UserIDS': { "$in": newID}}).count() > 0

If newID is not an array you should not use the $in operator. You can simply do find({'UserIDS': newID}).