How to append to bson object

I have an endpoint where users can filter a mongo collection using query parameters. If I have just one query parameter e.g. title, I can do this -

filter := bson.M{}

if params.Title != "" {
	filter = bson.M{"title": params.Title}
}

However, if I have more than one query parameter, I can't seem to get how to append to the bson object.

I tried this -

filter := []bson.M{}

if params.Title != "" {
	filter = append(filter, bson.M{"title": params.Title})
}

if params.Description != "" {
	filter = append(filter, bson.M{"description": params.Description})
}

but I got this error - cannot transform type []primitive.M to a BSON Document: WriteArray can only write a Array while positioned on a Element or Value but is positioned on a TopLevel

How do I solve this?

bson.M{} is underlined map[string]interface{} in go-mongo-driver. So if you need to add more elemnets, you can not append. Just assign that value to map's key as below.

    filter := bson.M{}
	
	if params.Title != "" {
		//filter = bson.M{"title": params.Title}
		filter["title"] = params.Title
	}

	if params.Description != "" {
		filter["description"] =  params.Description
	}

Consider a collection test with a document: { "_id" : 1, "Title" : "t-1", "Description" : "d-1" }
And, you can use the following:

title := "t-1"
description := ""    // or "d-1"
filter := bson.M{}

if Title != "" {
    filter["Title"] = title
}

if Description != "" {
    filter["Description"] = description
}

//fmt.Println(filter);

var result bson.M

collection := client.Database("test").Collection("test")
err := collection.FindOne(context.TODO(), filter).Decode(&result)
if err != nil {
    log.Fatal(err)
}

fmt.Printf("Found a single document: %+v\n", result)