Question

我正在努力建立对蒙戈的理解，但许多事情仍然是神秘的。我像这样进行聚合调用：

> db.my_collection_keys.aggregate([ { $project: {value: 1, gaa: {$literal: "baa"}}}])
{ "_id" : "_id", "value" : "dada_id((_id),(_id),(_id),(_id))", "gaa" : "baa" }
{ "_id" : "author", "value" : "(author)", "gaa" : "baa" }
{ "_id" : "available", "value" : "(available)", "gaa" : "baa" }
{ "_id" : "checkout", "value" : "(checkout)", "gaa" : "baa" }
{ "_id" : "language", "value" : "(language)", "gaa" : "baa" }
{ "_id" : "pages", "value" : "(pages)", "gaa" : "baa" }
{ "_id" : "published_date", "value" : "(published_date)", "gaa" : "baa" }
{ "_id" : "publisher_id", "value" : "(publisher_id)", "gaa" : "baa" }
{ "_id" : "title", "value" : "(title)", "gaa" : "baa" }
>

我所看到的看起来非常像我在集合上执行.find（）时看到的内容，所以我认为这将是一个集合，但类似

> db.my_collection_keys.aggregate([ { $project: {value: 1, gaa: {$literal: "baa"}}}]).find({$eq: {_id: "author"}})

不起作用。那么究竟是什么返回以及如何使用它呢？

（我知道将输出写入新集合的$ out关键字，但我对操作不太感兴趣，但理解。）

编辑：感谢@JohnnyHK我能够做到以下（作为概念证明）：

> var myCursor = db.my_collection_keys.aggregate([ { $project: {value: 1, gaa: {$literal: "baa"}}}])
> while (myCursor.hasNext()) {
 x = myCursor.next();
  print(tojson(x));
 db.dudu.insert(x);
}
... ... ... ... {
    "_id" : "_id",
    "value" : "dada_id((_id),(_id),(_id),(_id))",
    "gaa" : "baa"
}
{ "_id" : "author", "value" : "(author)", "gaa" : "baa" }
{ "_id" : "available", "value" : "(available)", "gaa" : "baa" }
{ "_id" : "checkout", "value" : "(checkout)", "gaa" : "baa" }
{ "_id" : "language", "value" : "(language)", "gaa" : "baa" }
{ "_id" : "pages", "value" : "(pages)", "gaa" : "baa" }
{ "_id" : "published_date", "value" : "(published_date)", "gaa" : "baa" }
{ "_id" : "publisher_id", "value" : "(publisher_id)", "gaa" : "baa" }
{ "_id" : "title", "value" : "(title)", "gaa" : "baa" }
WriteResult({ "nInserted" : 1 })
> db.dudu.find().pretty();
{
    "_id" : "_id",
    "value" : "dada_id((_id),(_id),(_id),(_id))",
    "gaa" : "baa"
}
{ "_id" : "author", "value" : "(author)", "gaa" : "baa" }
{ "_id" : "available", "value" : "(available)", "gaa" : "baa" }
{ "_id" : "checkout", "value" : "(checkout)", "gaa" : "baa" }
{ "_id" : "language", "value" : "(language)", "gaa" : "baa" }
{ "_id" : "pages", "value" : "(pages)", "gaa" : "baa" }
{ "_id" : "published_date", "value" : "(published_date)", "gaa" : "baa" }
{ "_id" : "publisher_id", "value" : "(publisher_id)", "gaa" : "baa" }
{ "_id" : "title", "value" : "(title)", "gaa" : "baa" }
>

将光标的文档放入集合中。我进一步收集， mongo做事的方式宁愿在聚合管道中添加另一个步骤。

Answer 1

如the shell docs中所述，aggregate会返回：

汇总管道操作最后阶段生成的文档的cursor

光标只提供对结果集的访问，并且不可查询。

要在显示时对结果进行后期过滤，您可以在管道中添加$match阶段：

db.my_collection_keys.aggregate([ 
    { $project: {value: 1, gaa: {$literal: "baa"}}},
    { $match: {$eq: {_id: "author"}})
]);

mongo db.mycollection.aggregate（...）返回什么样的东西？

1 个答案: