Sort mongo results if a value exists in the field or not

I have a Student struct which looks like this.

type Student struct {
	Name            string                         `json:"name" bson:"name"`
	Marks           int                            `json:"marks" bson:"marks"`
	Subjects        []string                       `json:"subjects" bson:"subjects"`
}

I am using opts.Sort to Sort the result. More on that

opts.Sort = bson.D{
	{Key: "marks", Value: -1},
}

I also want to sort the results by Subjects, in a way that, if for any Student, if the subject Math exist, it should be sorted on top (descending order), before sorting it by marks
I tried doing this

opts.Sort = bson.D{
    {Key: "subjects", Value: bson.M{"$in": "math"}},
	{Key: "marks", Value: -1},
}

I know this doesn't seem right because I am not passing 1 or -1 but I don't know how can I modify it to make it work.
What am I missing here?.

You can't do this with a single "simple" query.

Query the records sorted by marks, and do a second sorting in Go, by moving documents having "math" subject to the front.

If you need to do this only in MongoDB, you may redesign: e.g. you may add a boolean field to documents storing the information whether the student has "math", so you can easily include that in sorting.

Do note however that you may do this with the Aggregation framework. This is the query that would do what you need:

db.students.aggregate(
    {$addFields:{"hasMath": {$in:["math", "$subjects"]}}},
    {$sort:{hasMath: -1, marks: -1}}
)

What this does is essentially what I suggested: it adds a hasMath field, telling if the student has "math" in the subjects array, and then sorts documents first by hasMath descending, then by marks descending.

This is how you can do that in Go using the official mongo-go driver:

c := ... // Obtain students collection

pipe := []bson.M{
	{"$addFields": bson.M{
		"hasMath": bson.M{"$in": []any{"math", "$subjects"}},
	}},
	{"$sort": bson.D{
		{Key: "hasMath", Value: -1},
		{Key: "marks", Value: -1},
	}},
}

curs, err := c.Aggregate(ctx, pipe)
if err != nil {
	// Handle error
	panic(err)
}

var students []Student
if err := curs.All(ctx, &students); err != nil {
	// Handle error
	panic(err)
}