Get last submissions of a user

def get_user_last_submissions(self, limit=5, request=None):
        """ Get last submissions of a user """
        if request is None:
            request = {}

        request.update({"username": self._user_manager.session_username()})

        # Before, submissions were first sorted by submission date, then grouped
        # and then resorted by submission date before limiting. Actually, grouping
        # and pushing, keeping the max date, followed by result filtering is much more
        # efficient
        data = self._database.submissions.aggregate([
            {"$match": request},
            {"$group": {"_id": {"courseid": "$courseid", "taskid": "$taskid"},
                        "submitted_on": {"$max": "$submitted_on"},
                        "submissions": {"$push": {
                            "_id": "$_id",
                            "result": "$result",
                            "status" : "$status",
                            "courseid": "$courseid",
                            "taskid": "$taskid",
                            "submitted_on": "$submitted_on"
                        }},
            }},
            {"$project": {
                "submitted_on": 1,
                "submissions": {
                    # This could be replaced by $filter if mongo v3.2 is set as dependency
                    "$setDifference": [
                        {"$map": {
                            "input": "$submissions",
                            "as": "submission",
                            "in": {
                                "$cond": [{"$eq": ["$submitted_on", "$$submission.submitted_on"]}, "$$submission", False]
                            }
                        }},
                        [False]
                    ]
                }
            }},
            {"$sort": {"submitted_on": pymongo.DESCENDING}},
            {"$limit": limit}
        ])

        return [item["submissions"][0] for item in data]