Python 根据另一个字段中的重复项计算一个字段中的重复项的聚合函数

我使用mongoengine作为ORM和flask应用程序.模型类是这样定义的

class MyData(db.Document):
    task_id = db.StringField(max_length=50, required=True)
    url = db.URLField(max_length=500,required=True,unique=True)
    organization = db.StringField(max_length=250,required=True)
    val = db.StringField(max_length=50, required=True)

字段组织可以是重复的，我想得到与另一个字段中的值相关的重复计数.例如，如果mongodb中的数据如下

[{"task_id":"as4d2rds5","url":"https:example1.com","organization":"Avengers","val":"null"},
 {"task_id":"rfre43fed","url":"https:example1.com","organization":"Avengers","val":"valid"},
 {"task_id":"uyje3dsxs","url":"https:example2.com","organization":"Metro","val":"valid"},
 {"task_id":"ghs563vt6","url":"https:example1.com","organization":"Avengers","val":"invalid"},
 {"task_id":"erf6egy64","url":"https:example2.com","organization":"Metro","val":"null"}]

然后我使用

data = MyData.objects()

我想要一个像这样的回应

[{"url":"https:example1.com","Avengers":{"valid":1,"null":1,"invalid":1}},{"url":"https:example2.com",Metro":{"valid":1,"null":1,"invalid":0}}]

我试着

db.collection.aggregate([
  {
    "$group": {
      "_id": "$organization",
      "count": [
        {
          "null": {
            "$sum": 1
          },
          "valid": {
            "$sum": 1
          },
          "invalid": {
            "$sum": 1
          }
        }
      ]
    }
  }
])

但我犯了个错误

The field 'count' must be an accumulator object