I have following data in my mongoDb database and I want to find the student from the array of students objects by iterating on all my students field of my database with the id the students array objects have.

I want to do it with the help of mongoose please help me find the solution as I am unable to find it.

Here is my database:

[
{
    "_id": "637ddf6d68a8284187a4d7e7",
    "college": "MIT"
    "students": [
        {
            "name": "Morgan Freeman",
            "image": "/public/morg.jpg",
            "_id": "637ddf6d68a8284187a4d7e8"
        },
        {
            "name": "John Smith",
        "image": "/public/john.jpg",
            "_id": "637ddf6d68a8284187a4d7e9"
        }
    ]
},

{
    "_id": "637ddf6d68a8284187a4dfhd",
    "college": "DOT"
    "students": [
        {
            "name": "Windy rona",
            "image": "/public/windy.jpg",
            "_id": "637ddf6d68a8284187a4dvh3"
        },
        {
            "name": "Richard",
        "image": "/public/richard.jpg",
            "_id": "637ddf6d68a8284187a4duhd"
        }
    ]
},

]

I tried using:

 database.find({}).select('students').where('_id':req.params.id)

but it didnot work

CodePudding user response：

Well if I understand your question you want to filter the result of the student array that match with the _id of the student that it providing

an aggregation with filter the project must help you.

here an example:

database.aggregate([
  {
    "$match": {
      "students._id": "637ddf6d68a8284187a4dvh3"
    },
    
  },
  {
    $project: {
      students: {
        $filter: {
          input: "$students",
          as: "student",
          cond: {
            $eq: [
              "$$student._id",
              "637ddf6d68a8284187a4dvh3"
            ]
          }
        }
      }
    }
  },
  {
    "$unwind": "$students"
  }
])

this will give you the following result:

[
  {
    "_id": "637ddf6d68a8284187a4dfhd",
    "students": 
      {
        "_id": "637ddf6d68a8284187a4dvh3",
        "image": "/public/windy.jpg",
        "name": "Windy rona"
      }
  }
]

CodePudding user response：

Option 1) Aggregation/$filter mongodb 3.2 Maybe something like this:

db.collection.aggregate([
{
 $match: {
  "students._id": "637ddf6d68a8284187a4duhd"
}
},
{
"$addFields": {
  "students": {
    "$filter": {
      "input": "$students",
      "as": "s",
      "cond": {
        "$eq": [
          "$$s._id",
          "637ddf6d68a8284187a4duhd"
        ]
      }
    }
  }
  }
 },
 {
   $unwind: "$students"
 },
{
"$project": {
  name: "$students.name",
  image: "$students.image",
  _id: "$students._id"
 }
 }
 ])

Exaplained:

1. Match the students._id document ( good to index this field )
2. $filter only students with the matched _id
3. $unwind the students array
4. project only the neccesary fields

Playground

Option 2) Find ( find first matching in a document , it will not show if there is other matching with this _id in the same document ):

db.collection.find({
  "students._id": "637ddf6d68a8284187a4duhd"
},
{
  "students": {
    "$elemMatch": {
      _id: "637ddf6d68a8284187a4duhd"
    }
  }
})

Explained:

1. Match documents if any _id
2. Project only the matching subelement

Playground 2