mongodb查找函数返回一个空数组结果

Question

我有两个mongodb收集员工和部门如下 员工集合：

{
"_id" : ObjectId("58c4a35ac2e604024321788e"),
"id_employee" : 19,
"employee_name" : "Lene Vestergaard Hau",
"employee_address" : "Allerton",
"hours" : 279,
"id_department" : 1,
"projects" : [
    285,
    453,
    499,
    804,
    956
],
"Children" : [
    {
        "id_child" : 38,
        "child_name" : "Caroline Herschel"
    }
]

}

部门收集：

{
"_id" : ObjectId("58c49c48c2e669d6555aa5a7"),
"id_department" : 1,
"department_name" : "dept1",
"projects" : [
    {
        "id_project" : 285,
        "project_name" : "1project 285",
        "duration" : "211"
    },
    {
        "id_project" : 453,
        "project_name" : "1project453",
        "duration" : "214"
    },
    {
        "id_project" : 499,
        "project_name" : "1project499",
        "duration" : "224"
    },
    {
        "id_project" : 804,
        "project_name" : "1project804",
        "duration" : "217"
    }
    {
        "id_project" : 956,
        "project_name" : "1project956",
        "duration" : "217"
    }

] 我正在尝试执行查找功能以获取有关的所有详细信息 员工的项目。我正在使用的java代码：

coll.aggregate(Arrays.asList(
        Aggregates.match(Filters.eq("id_employee",19)),
        Aggregates.unwind("$projects"),
        Aggregates.lookup("department", "projects",               "projects.id_project", "lookupData")

)).forEach(printBlock);
}

上面的代码将lookupData作为空列表返回。有人可以帮忙吗？