python - MongoEngine:嵌入式文档中的数组位置投影

Question

我正在尝试使用 MongoEngine 在 python 中重现以下查询，但没有成功。

数据和查询的原始来源: http://docs.mongodb.org/manual/reference/projection/positional/#prj._S_

查询基本上返回嵌入文档中匹配的第一个元素，而不是整个嵌入文档本身。

我的代码:

from mongoengine import *
connect('test')


class Student(Document):
    semester = IntField()
    grades = ListField(EmbeddedDocumentField('Grade'))


class Grade(EmbeddedDocument):
    value = FloatField()

    def __str__(self):
        return "%s" % self.value

student_1 = Student(semester=1, grades=[Grade(value=70), Grade(value=87), Grade(value=90)]).save()
student_2 = Student(semester=1, grades=[Grade(value=90), Grade(value=88), Grade(value=92)]).save()

result = Student.objects(semester=1, grades__value__gte=85).only("grades.$")

for r in result:
    print r.grades

当我这样做时，出现错误:

Traceback (most recent call last):
  File "test_me_so.py", line 21, in <module>
    result = Student.objects(semester=1, grades__value__gte=85).only("grades.$")
  File "/.../lib/python2.7/site-packages/mongoengine/queryset.py", line 1225, in only
    return self.fields(**fields)
  File "/.../lib/python2.7/site-packages/mongoengine/queryset.py", line 1271, in fields
    fields = self._fields_to_dbfields(fields)
  File "/.../lib/python2.7/site-packages/mongoengine/queryset.py", line 1289, in _fields_to_dbfields
    field = ".".join(f.db_field for f in QuerySet._lookup_field(self._document, field.split('.')))
  File "/.../lib/python2.7/site-packages/mongoengine/queryset.py", line 1289, in <genexpr>
    field = ".".join(f.db_field for f in QuerySet._lookup_field(self._document, field.split('.')))
AttributeError: 'str' object has no attribute 'db_field'

如有任何帮助，我们将不胜感激!

Answer 1

我不确定 mongoengine 的语法，但我认为您不需要“唯一”位。

$位置指示器通常会从数组中抓取相关值，听上去会涉及到。