MongoDB's Aggregation Framework: project only matching element of an array

Question

I have a "class" document as:

{
className: "AAA",
students: [
   {name:"An", age:"13"},
   {name:"Hao", age:"13"},
   {name:"John", age:"14"},
   {name:"Hung", age:"12"}
   ]
}

And i want to get the student who has name is "An", get only matching element in array "students". I can do that with function find() as:

>db.class.find({"students.name":"An"}, {"students.$":true})
{
"_id" : ObjectId("548b01815a06570735b946c1"),
"students" : [ 
    {
        "name" : "An",
        "age" : "13"
    }
]}

It's fine, but when i do the same with Aggregation as following, it get error:

db.class.aggregate([
   {$match:{"students.name":'An'}},
   {$project:{"students.$":true}}
])

Error is:

uncaught exception: aggregate failed: {
    "errmsg" : "exception: FieldPath field names may not start with '$'.",
    "code" : 16410,
    "ok" : 0
}

Why? I can't use "$" for array in $project operator of aggregate() while can use this one in project operator of find().

Answer 1

From the docs:

Use $ in the projection document of the find() method or the findOne() method when you only need one particular array element in selected documents.

The positional operator $ cannot be used in an aggregation pipeline projection stage. It is not recognized there.

This makes sense, because, when you execute a projection along with a find query, the input to the projection part of the query is a single document that has matched the query.The context of the match is known even during projection. So for each document that matches the query, the projection operator is applied then and there before the next match is found.

db.class.find({"students.name":"An"}, {"students.$":true})

In case of:

db.class.aggregate([
   {$match:{"students.name":'An'}},
   {$project:{"students.$":true}}
])

The aggregation pipeline is a set of stages. Each stage is completely unaware and independent of its previous or next stages. A set of documents pass a stage completely before being passed on to the next stage in the pipeline. The first stage in this case being the $match stage, all the documents are filtered based on the match condition. The input to the projection stage is now a set of documents that have been filtered as part of the match stage.

So a positional operator in the projection stage makes no sense, since in the current stage it doesn't know on what basis the fields had been filtered. Therefore, $ operators are not allowed as part of the field paths.

Why does the below work?

db.class.aggregate([
     { $match: { "students.name": "An" },
     { $unwind: "$students" },
     { $project: { "students": 1 } }
])

As you see, the projection stage gets a set of documents as input, and projects the required fields. It is independent of its previous and next stages.

Answer 2

Try using the unwind operator in the pipeline: http://docs.mongodb.org/manual/reference/operator/aggregation/unwind/#pipe._S_unwind

Your aggregation would look like

db.class.aggregate([
     { $match: { "students.name": "An" },
     { $unwind: "$students" },
     { $project: { "students": 1 } }
])