我是蒙哥的新蜜蜂。下面是我正在执行的聚合操作 mongodb 贝壳。但是在我的java项目聚合中，我不能给出 $toObjectId . 请纠正我遗漏了什么。 db shell query ```
db.getCollection('UserData').aggregate([

{
$project : {
"username" : "$username",
"beneficiaries" : "$beneficiaries"

 }   

},
{
$unwind : {
path : "$beneficiaries",
preserveNullAndEmptyArrays: true
}
},
{
$project : {
"username" : "$username",
"beneficiaries" : "$beneficiaries",
---- dont know how to give $toObjectId in java ProjectionOperation .
"beneficiaryStudId" : { $toObjectId : "$beneficiaries.studentId" }
}
},
{
$lookup:
{
from: "StudentProfileData",
localField: "beneficiaryStudId",
foreignField: "_id",
as: "studProfile"
}
}
])
`Java code Projection Operation`
ProjectionOperation projectUserAndBeneficiaries = Aggregation.project()
.andExpression("username").as("username")
.andExpression("beneficiaries").as("beneficiaries");

    ProjectionOperation projectUserAndOtherDetails = Aggregation.project()
              .andExpression("username").as("username")
              .andExpression("beneficiaries").as("beneficiaries")

---- How to give $toObjectId in projection operation .andExpression("beneficiaries.studentId").as("beneficiaryStudId");

    LookupOperation lookupOperation = LookupOperation.newLookup().
            from("StudentProfileData").
            localField("beneficiaryStudId").
            foreignField("_id").
            as("studProfile");
    Aggregation agg = Aggregation.newAggregation(projectUserAndBeneficiaries, unwindBeneficiars,  
            projectUserAndOtherDetails
            ,lookupOperation);
    AggregationResults<UserAndStudentData> output 
      = mongotemplate.aggregate(agg, "UserData", UserAndStudentData.class);

样本输出

Output in db shell
{
"_id" : ObjectId("5d2f08574de2690001c281ac"),
"username" : "ks241@goo.com",
"beneficiaries" : {
"studentId" : "5d2f0e9c3bcf3e0001a7e562",
"mcBeneficiaryId" : "597418",
"enabled" : true
},
"beneficiaryStudId" : ObjectId("5d2f0e9c3bcf3e0001a7e562"),
"studProfile" : [
{
"_id" : ObjectId("5d2f0e9c3bcf3e0001a7e562"),
"lastName" : "Sharma",
"firstName" : "Kapil",
"studentRegisterCustomFieldValues" : [
{
"bcfdValue" : "One",
"bcfdName" : "Year"
}
],
"gender" : "M",
"merchantId" : "38788943"
}
]
}

在java中
如果我在上面的java聚合投影查询中添加$toobjectid并运行它，studprofile数组总是空的，它生成的值与db shell相同。