bson：“-”在插入时不会解组并忽略

I have been facing some difficulties with $lookup:

My struct:

type Item struct {
    ID          bson.ObjectId   `json:"id" bson:"_id"`
    Name        string          `json:"name" bson:"name"`
    Description string          `json:"description" bson:"description"`
    CoverImage  bson.ObjectId   `json:"cover_image,omitempty" bson:"cover_image,omitempty"`
    GalleryList []*File
}

pipeline := []bson.M{
    bson.M{
        "$lookup": bson.M{
            "from":         "files",
            "localField":   "cover_image",
            "foreignField": "_id",
            "as":           "gallerylist",
    },
}

So, what I want to do is use $lookup to get structs with matching ID on collection "files" with the "cover_image" field ID.

Until here it's ok, the problem is:

If 'bson:"-"' is on GalleryList, the pipeline don't unmarshall on the field

Ex: GalleryList []*File 'bson:"-"'

And I don't want GalleryList to be stored inside mongo.

Is there anything that I can to GalleryList don't be inserted with the rest of the struct.

Does I really need to remove this on the frontend or remove manually before inserting on mongo?

Thanks!