Swift：通过比较类型过滤协议数组(Swift: filter protocol array by comparing types)

 我可以看到这个命令的一些问题： 
 
db.location.aggregate([
 {$match : { FirstName: { $nin : [" "] } } },
 {$group: {_id : "$DeviceId" } },
 {$project: { "DeviceId" : 1 } }
])
 
 
 
 $match阶段与您问题中提供的文档不匹配，因为您提供的文档具有"FirstName" : " "因此该文档将匹配$in而不是$nin 
 
 
 如果$match阶段更改为$in那么它将发出一个文档，分组阶段将有一些工作，但分组阶段只配置为返回_id属性，因此它返回以下中间文档： 
{
    "_id" : "89984320001499681816"
}
 
 
 $project阶段尝试投影"DeviceId"但分组阶段的结果仅包含_id属性，因此您无法从该文档投影DeviceId属性。 
 
 
 当您的聚合管道到达forEach函数时，文档中没有DeviceId，FirstName或LastName属性。 
 
 
 要进一步了解聚合管道内发生的情况，您可以运行此查询： 
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$project: { "DeviceId" : 1, FirstName: 1, LastName: 1 } }
]).forEach(
  function(obj) {
      print(obj);
  }
);
 
 或者，只需自己运行管道中的每个阶段并查看每个状态的输出，因为这将成为下一阶段的输入。 所以，例如： 
 
 这个命令...... 
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } }
])
 
 ...返回： 
 
{
    "_id" : ObjectId("59d13b64c26584cd8b7a15c8"),
    "CreationDateTime" : ISODate("2017-09-26T06:39:29.105Z"),
    "DeviceId" : "89984320001499681816",
    "UserId" : UUID("bca0db12-2246-49a5-8703-b03fee45e50f"),
    "FirstName" : " ",
    "LastName" : ""
}
 
 这个命令...... 
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$group: {_id : "$DeviceId" } }
])
 
 ...返回： 
 
{
    "_id" : "89984320001499681816"
}
 
 这个命令...... 
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$group: {_id : "$DeviceId" } },
 {$project: { "DeviceId" : 1 } }
])
 
 ...返回： 
 
{
    "_id" : "89984320001499681816"
}
 
 这应该清楚地表明，一旦添加$group阶段，您将丢失DeviceId，FirstName和LastName属性，因此它们无法在forEach函数中打印。 

I can see a few issues with this command:
 
db.location.aggregate([
 {$match : { FirstName: { $nin : [" "] } } },
 {$group: {_id : "$DeviceId" } },
 {$project: { "DeviceId" : 1 } }
])
 
 
 
The $match stage will not match the document supplied in your question because the document you supplied has "FirstName" : " " so that document would be matched by $in but not by $nin
 
 
If the $match stage was changed to $in then it would emit a document and the grouping stage would have something to work on but the grouping stage is only configured to return an _id attribute so it returns the following intermediate document:
 
{
    "_id" : "89984320001499681816"
}
 
 
The $project stage attempts to project "DeviceId" but the result of the grouping stage contains only an _id attribute so you cannot project a DeviceId attribute from that document.
 
 
By the time your aggregation pipeline reaches the forEach funciton there is no DeviceId, FirstName or LastName attribute in the documents.
 
 
To further understand what's happening inside the aggregation pipeline you could run this query:
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$project: { "DeviceId" : 1, FirstName: 1, LastName: 1 } }
]).forEach(
  function(obj) {
      print(obj);
  }
);
 
Or, just run each stage in the pipeline on its own and review the output of each state since that becomes the input to the next stage. So, for example:
 
This command ...
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } }
])
 
... returns:
 
{
    "_id" : ObjectId("59d13b64c26584cd8b7a15c8"),
    "CreationDateTime" : ISODate("2017-09-26T06:39:29.105Z"),
    "DeviceId" : "89984320001499681816",
    "UserId" : UUID("bca0db12-2246-49a5-8703-b03fee45e50f"),
    "FirstName" : " ",
    "LastName" : ""
}
 
This command ...
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$group: {_id : "$DeviceId" } }
])
 
... returns:
 
{
    "_id" : "89984320001499681816"
}
 
This command ...
 
db.location.aggregate([
 {$match : { FirstName: { $in : [" "] } } },
 {$group: {_id : "$DeviceId" } },
 {$project: { "DeviceId" : 1 } }
])
 
... returns:
 
{
    "_id" : "89984320001499681816"
}
 
This should make clear that once you add in the $group stage you lose the DeviceId, FirstName and LastName attributes hence they are not available for you to print in the forEach function.