Mongodb 嵌套数组的所有元素都不是相同值的查询集合

我的注册数据结构如下：

event: "Event A",
level: "A",
field: "Main",
buyer: ["thisuser@gmail.com"],
players: [
  { 
  first: "Bob",
  last: "Baker",
  email: "bobbaker@gmail.com",
  myid: 1234567,
  signed: false
  },
  {
  first: "John",
  last: "Doe",
  email: "johndoe@who.com",
  myid: 4325678,
  signed: true
  }
]

我正在尝试对注册参加不同级别活动的玩家进行查询。到目前为止，我已经：

db.registrations.aggregate(     
[       
{ $unwind: '$players' },       
{         
  $group: {           
    _id: { myid: '$players.myid' },           
    events: {             
      $push: {               
        event: '$event',               
        level: '$level',               
        field: '$field',               
        buyer: ['$buyer']             
      }           
    }         
  }       
},       
{ $match: { events: { $size: 2 }, 'events.level': 'A' } }     
])

我在想，如果我能够将所有events.level与events数组中的第一条记录进行比较，那么聚合的最后一行就可以工作了。我只是不知道怎么做。我希望数据输出如下：

{ 
"_id" : { "myid" : 1081245 }, 
"events" : [ 
  { "event" : "Women's AA", "level" : "AA", "field" : "PAL", "buyer" : [ "email@osu.edu" ] }, 
  { "event" : "COED A", "level" : "A", "field" : "Main", "buyer" : [ "ngruber@cdeducation.org" ] } ] }
{ "_id" : { "myid" : 1081202 }, 
"events" : [ 
  { "event" : "Women's BB", "level" : "BB", "field" : "PAL", "buyer" : [ "amothermail@gmail.com" ] }, 
  { "event" : "COED A", "level" : "A", "field" : "Main", "buyer" : [ "emailing@gmail.com" ] } ] }
{ "_id" : { "myid" : 1081196 }, 
"events" : [ 
  { "event" : "Men's A", "level" : "A", "field" : "PAL", "buyer" : [ "myemail@outlook.com" ] }, 
  { "event" : "COED BB", "level" : "BB", "field" : "Main", "buyer" : [ "hisemail@gmail.com" ] } ] }
{ "_id" : { "myid" : 1080246 }, 
"events" : [ 
  { "event" : "Men's A", "level" : "A", "field" : "PAL", "buyer" : [ "dude@outlook.com" ] }, 
  { "event" : "COED BB", "level" : "BB", "field" : "Main", "buyer" : [ "fella@gmail.com" ] } ] }

我尝试将聚合的最后一行更改为：

{ $match: { events: { $size: 2 }, {$not: {'events.level': '$first'} }} }

---

如何执行此操作？

在上面的示例数据中，前两个文档不包含

事件。级别
在事件数组的第一个位置等于
A
，那么返回的原因是什么？在上面的示例数据中，前两个文档不包含
事件。级别
在事件数组的第一个位置等于
A
，那么返回的原因是什么返回？