mongodb aggregate get array of dic value become new field-CodePudding

After I execute aggregate, I have following data:

{
    "_id" : ObjectId("61ea385114e22e000c58a502"),
    "stu_id" : "h224566378",
    "stu_inf" : [ 
        {
            "_id" : ObjectId("61ea3a35c34a334514e7e8df"),
            "stu_id" : "h224566378",
            "gender" : "male",
            "class" : "A",
        }
    ]
}

If I want data to be the following:

{
    "_id" : ObjectId("61ea385114e22e000c58a502"),
    "stu_id" : "h224566378",
    "stu_class" : "A" 
}

How to use "aggregate" to get my needs?
I want the "class value" in the extracted "stu_inf" to become the value of the new field.

I try following way, it can get the result:

db.collection_name.aggregete([
    ...
    {"$project": {"stu_id": 1, "stu_class": "$stu_inf.class"}},
    {"$unwind": "$stu_class"}
])

but I wonder if there is any other way?

CodePudding user response：

Use $first

db.collection.aggregate([
  {
    $project: {
      "stu_id": 1,
      "stu_class": {
        $first: "$stu_inf.class"
      }
    }
  }
])

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