Pymongo-aggregatie - pythonlijst doorgeven voor aggregatie

Je zou het volgende kunnen proberen:

key_list =  ["animal.dog", "animal.dog.tail", "animal.cat", "tree", "fruits", "timestamp"]
match = { "$match": { "cid": ObjectId("57fe39972b8dbc1387b20913") } }
project = { "$project": {} }
group = { "$group": {} }

for item in key_list:
    if item == "timestamp":
        project["$project"]["day"] = { "$substr": ["$"+item,  0, 10] }
        group["$group"]["_id"] = "$day"
        break
    sum = {"$sum": ""}
    sum["$sum"] = "$"+item.replace(".", "_")
    project["$project"][item.replace(".", "_")] = "$"+item
    group["$group"][item.replace(".", "_")] = sum

pipeline = [match, project, group]