Filter array based on value in nested array mongodb

Question

I have the following model:

{
    "_id" : ObjectId("...some id"),
    "lessons" : [
        {
            "date" : ISODate("2019-09-23T16:00:00.000+02:00"),
            "subject" : [
                "Javascript",
                "Order"
            ],
            "price" : 60
        },
        {
            "date" : ISODate("2019-09-24T16:00:00.000+02:00"),
            "subject" : [
                "Javascript"
            ],
            "price" : 120
        }
    ]
}

I would like to filter "lessons" array based on value in nested "subject" array. Filter i mean remove whole object from array "lessons" if for example value in nested array "subject" is not equeal to value "Order".

Expected output (remove object, becasuse nested array contains "Order" value):

{
    "_id" : ObjectId("...some id"),
    "lessons" : [
        {
            "date" : ISODate("2019-09-24T16:00:00.000+02:00"),
            "subject" : [
                "Javascript"
            ],
            "price" : 120
        }
    ]
}

I've tried use $filter pipeline but it overwrite whole array "lessons".

db.students.aggregate([
  {
    $project: {
        "lessons": {
            $filter: {
                input: "$lessons.subject",
                as: "subject",
                cond: {
                    $ne: [
                        "$$subject",
                        "Order"
                    ]

                }
            }
        }
    }  
  },
]);

Answer 1

What we can do here is:

• first use aggregation stage $unwind on the field lessons to treat them as individual documents.
• Then have a $match on subjects field (array) with the condition.
• And finally $group by _id and push lessons back via $push

Query:

db.collection.aggregate([
    { $unwind: "$lessons" },
    { $match: { "lessons.subject": { $ne: "Order" } } },
    {
      $group: {
        _id: "$_id",
        lessons: { $push: "$lessons" }
      }
    }
  ]).pretty();

Output:

{
    "_id" : ObjectId("5d6d0d401ce4f18674c11053"),
    "lessons" : [
        {
            "date" : ISODate("2019-09-24T14:00:00Z"),
            "subject" : [
                "Javascript"
            ],
            "price" : 120
        }
    ]
}