如何在MongoEngine中引用一对多-在模式和聚合输出中（新手）

Question

我正在尝试输出一个班级（来自班级模型）及其引用的学生-学习mongoengine和mongodb。下面的代码给我错误。

Expected 'pipeline' to be BSON docs (or equivalent), but got []

我确定这对那些知道mongo和mongoengine的人来说是显而易见的。任何帮助（或朝着正确的方向推动）：）提前感谢

import urllib
from mongoengine import *


connect(db=DB_NAME, username=DB_USER, password=DB_PASSWORD,
        host=DB_URI)

class Students(Document):
    student_id = IntField(unique=True)  
    name = StringField(max_length=50)
    age = IntField(max_length=2)
    gender = StringField(choices=('male', 'female'))


class Classes(Document):
    class_id = IntField(required=True, unique=True)  # 1576407600000
    student_roster = ListField(ReferenceField(Students))


Students.objects.insert([
    Students(name="John", student_id=425736, age=10, gender="male"),
    Students(name="Mary", student_id=114391, age=9, gender="female")
])

Classes(class_id=1576407600000, student_roster=[425736, 114391]).save()


# gives pipeline error
c = Classes.objects.aggregate([
    {'$lookup': {'from': 'students',
            'localField' : 'student_roster',
            'foreignField' : 'student_id',
            'as': 'studentData'
    }
}
])
list(c)

Answer 1

如果我指的是the docs：

class Person(Document):
    name = StringField()

Person(name='John').save()
Person(name='Bob').save()

pipeline = [
    {"$sort" : {"name" : -1}},
    {"$project": {"_id": 0, "name": {"$toUpper": "$name"}}}
    ]
data = Person.objects().aggregate(*pipeline)
assert data == [{'name': 'BOB'}, {'name': 'JOHN'}]

您使用的是objects成员，而不是objects()成员，并且传递了list，希望将其解压缩为dict参数（*pipeline，您将等价于pipeline）