Mapping a nested struct with mongo'd db .Decode(&dto)

I have a model that creates a JSON document with no issue but retrieving it results in the nested JSON object being empty.

func (r *CourseRepo) GetCourseById(ctx context.Context, id string) (Course, error) {
	clog := log.GetLoggerFromContext(ctx)

	var course Course

	objID, err := primitive.ObjectIDFromHex(id)
	if err != nil {
		return course, err
	}

	filter := bson.M{"_id": objID}

	err = r.collection.FindOne(ctx, filter).Decode(&course)
	if err != nil {
		clog.ErrorCtx(err, log.Ctx{"msg": "an error occurred while finding a course"})

		return course, err
	}

	return course, nil
}

Struct

type Course struct {
	ObjectId    primitive.ObjectID `bson:"_id, omitempty"`
	Id          string             `json:"id"`
	Title       string             `json:"title"`
	Description string             `json:"description"`
	Lessons     string             `json:"lessons"`
	Duration    string             `json:"duration"`
	Details     struct {
		Title             string `json:"title"`
		Instructor        string `json:"instructor"`
		Introduction      string `json:"introduction"`
		Learn             string `json:"learn"`
		Topics            string `json:"topics"`
		Prerequisites     string `json:"prerequisites"`
		Goal              string `json:"goal"`
		AdditionalDetails string `json:"additionalDetails"`
		HighLevelOverview string `json:"highLevelOverview"`
	} `json:"course_details"`
}

result

{
    "data": {
        "ObjectId": "64953ac1bf06bfdd7936cad8",
        "id": "",
        "title": "Java Algorithms",
        "description": "An awesome course",
        "lessons": "4",
        "duration": "10 hours",
        "course_details": {
            "title": "",
            "instructor": "",
            "introduction": "",
            "learn": "",
            "topics": "",
            "prerequisites": "",
            "goal": "",
            "additionalDetails": "",
            "highLevelOverview": ""
        }
    },
    "metadata": "none"
}

From what I read .Decode should map the nest values too?

JSON? But the Go MongoDB driver works with BSON

The struct tags are used to define how the Go struct fields should be mapped to the MongoDB document fields.
In your Course struct, you are using json tags, but the Decode method from the MongoDB driver in Go uses the bson tags to map the document fields to the struct fields.

To solve this issue, you should add bson tags to your struct fields, including the nested struct, to instruct the MongoDB driver how to decode the document into your struct.

type Course struct {
    ObjectId    primitive.ObjectID `bson:"_id,omitempty" json:"ObjectId"`
    Id          string             `bson:"id" json:"id"`
    Title       string             `bson:"title" json:"title"`
    Description string             `bson:"description" json:"description"`
    Lessons     string             `bson:"lessons" json:"lessons"`
    Duration    string             `bson:"duration" json:"duration"`
    Details     struct {
        Title             string `bson:"title" json:"title"`
        Instructor        string `bson:"instructor" json:"instructor"`
        Introduction      string `bson:"introduction" json:"introduction"`
        Learn             string `bson:"learn" json:"learn"`
        Topics            string `bson:"topics" json:"topics"`
        Prerequisites     string `bson:"prerequisites" json:"prerequisites"`
        Goal              string `bson:"goal" json:"goal"`
        AdditionalDetails string `bson:"additionalDetails" json:"additionalDetails"`
        HighLevelOverview string `bson:"highLevelOverview" json:"highLevelOverview"`
    } `bson:"course_details" json:"course_details"`
}

Note that you can have both bson and json tags on the same field. The bson tags are used when interacting with MongoDB (e.g., when calling .Decode()), and the json tags are used when marshaling/unmarshaling to/from JSON format.

Also, make sure that the field names in your bson tags match the field names in your MongoDB documents. For example, if the field in the MongoDB document is named courseDetails instead of course_details, you should change the bson tag to bson:"courseDetails".