MongoDB的骨料/組/總和的查詢翻譯成pymongo查詢

Question

我有goals集合看起來像這樣的一組條目：

{"user": "adam", "position": "attacker", "goals": 8} 
{"user": "bart", "position": "midfielder", "goals": 3} 
{"user": "cedric", "position": "goalkeeper", "goals": 1} 

我想計算的所有目標的總和。在MongoDB shell中，我這樣做：

> db.goals.aggregate([{$group: {_id: null, total: {$sum: "$goals"}}}]) 
{ "_id" : null, "total" : 12 } 

現在我想在Python中使用pymongo來做同樣的事情。我嘗試使用db.goals.aggregate()和db.goals.group()，但迄今爲止沒有成功。

非工作查詢：

> query = db.goals.aggregate([{"$group": {"_id": None, "total": {"$sum": "$goals"}}}]) 
{u'ok': 1.0, u'result': []} 

> db.goals.group(key=None, condition={}, initial={"sum": "goals"}, reduce="") 
SyntaxError: Unexpected end of input at $group reduce setup 

任何想法如何做到這一點？

Answer 1

只需使用管道與聚合。

pipe = [{'$group': {'_id': None, 'total': {'$sum': '$goals'}}}] 
db.goals.aggregate(pipeline=pipe) 

Out[8]: {u'ok': 1.0, u'result': [{u'_id': None, u'total': 12.0}]}