为什么它返回 null 而不是 value？

Question

> db.patients.findOne()

{ "_id" : ObjectId("603fc797f0af296be54caecc"), "name" : "法希姆", “年龄”：29， “疾病摘要”：“f_26_ds” }

<块引用>

db.diseaseSummary.findOne() {“_id”：“f_26_ds”，“疾病”：[“发烧”，“咳嗽”，“电晕”]} var dsd = db.patients.findOne().diseaseSummary 数据表 f_26_ds db.diseaseSummary.findOne({_id: dsd }) 空

> db.patients.findOne()
{
        "_id" : ObjectId("603fc797f0af296be54caecc"),
        "name" : "Fahim ",
        "age" : 29,
        "diseaseSummary" : "f_26_ds "
}
> db.diseaseSummary.findOne()
{ "_id" : "f_26_ds", "diseases" : [ "fever ", "cough ", "corona " ] }
> var dsd = db.patients.findOne().diseaseSummary
> dsd
f_26_ds
> db.diseaseSummary.findOne({_id: dsd } )
null
> ?????? why "null" comes