Java 在集合中通过排序获取唯一文档

我有一个mongo收藏“Student”，里面有以下文件

`{
    "_id" : ObjectId("5d006ed8506fe860333c5aa2"),
    "studentId" : "123",
    "examName" : "abc",
    "marks" : 56,
    "isLatest" : 34343,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-09T07:02:00.650Z")
}
{
    "_id" : ObjectId("5d006ed8506fe860333cwewew2"),
    "studentId" : "123",
    "examName" : "abc",
    "marks" : 57,
    "isLatest" : 45454,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}
{
    "_id" : ObjectId("5d006ed8506fe860333c5aa5"),
    "studentId" : "123",
    "examName" : "def",
    "marks" : 56,
    "isLatest" : 4434,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}
{
    "_id" : ObjectId("5d006ed8506fe860333c5aa8"),
    "studentId" : "123",
    "examName" : "def",
    "isLatest" : 1,
    "examStatus" : "Under progress",
    "addedOn" : ISODate("2019-06-11T07:02:00.650Z")
}
{
    "_id" : ObjectId("5d00708a506fe860333c5ad9"),
    "studentId" : "456",
    "examName" : "def",
    "marks" : 56,
    "isLatest" : 1,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}`

现在我想在examStatus完成的examName上获取文档的最新唯一数据。例如，对于examName:def，查询应返回以下文档

`{
    "_id" : ObjectId("5d00708a506fe860333c5ad9"),
    "studentId" : "456",
    "examName" : "def",
    "marks" : 56,
    "isLatest" : 1,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}`

对于testName abc，它应该返回

`{
    "_id" : ObjectId("5d006ed8506fe860333cwewew2"),
    "studentId" : "123",
    "examName" : "abc",
    "marks" : 57,
    "isLatest" : 45454,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}`

如果检查状态为“正在进行”的文档更改为“已完成”，则如下所示

{
    "_id" : ObjectId("5d006ed8506fe860333c5aa8"),
    "studentId" : "123",
    "examName" : "def",
    "marks" : 99,
    "isLatest" : 1,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-11T07:02:00.650Z")
}

然后，examName def和examStatus completed的学生输出应返回我

{
    "_id" : ObjectId("5d00708a506fe860333c5ad9"),
    "studentId" : "456",
    "examName" : "def",
    "marks" : 56,
    "isLatest" : 1,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-10T07:02:00.650Z")
}
{
    "_id" : ObjectId("5d006ed8506fe860333c5aa8"),
    "studentId" : "123",
    "examName" : "def",
    "marks" : 99,
    "isLatest" : 1,
    "examStatus" : "Completed",
    "addedOn" : ISODate("2019-06-11T07:02:00.650Z")
}

到目前为止，我尝试了以下查询

db.getCollection'Student'。聚合[ {$sort:{addedOn:-1}}， {$match:{examName:def}} ]试试这种方法

db.Student.find({examName:"def", examStatus:"Completed"}).sort({addedOn : -1}).limit(1)

否则，要获取最新值，可以按ObjectId排序。之所以可以这样做，是因为ObjectId的4字节值表示自Unix纪元以来的秒数

db.Student.find({examName:"def", examStatus:"Completed"}).sort({_id : -1}).limit(1)

更新

我认为你不能通过一个查询就做到这一点。尝试使用两个查询

var student = db.Student.findOne({examName:"abc", examStatus:"Completed"})
db.Student.find({"addedOn" : {$gte : student.addedOn}, "examName" : {$eq : student.examName}, "examStatus" : {$eq : student.examStatus}});

---

这将始终只给我一个文档。id为的文档将examstatus从“正在处理”更改为“已完成”，然后输出应该是具有以下id的文档`{{u id:ObjectId5d00708a506fe860333c5ad9}{{u id:ObjectId5d006ed8506fe860333c5aa8}如果需要多个文档，只需删除limit1函数即可。然后它会给出所有相关的文件。然后它会给出所有三个文件，其中两个文件是给学生的。ID:123但对于这一点，我只需要一个带有最新附录的文件。现在我明白你的问题了。我已经编辑了答案。试试看。