How Find function's generic map was created?

Question

I am looking at this example. I would never coome up with solution like this,I would go for bson.raw.

type Movie struct {
    ID        bson.ObjectId `json:"id" bson:"_id,omitempty"`
    Name      string        `json:"name" bson:"name"`
    Year      string        `json:"year" bson:"year"`
    Directors []string      `json:"directors" bson:"directors"`
    Writers   []string      `json:"writers" bson:"writers"`
    BoxOffice BoxOffice     `json:"boxOffice" bson:"boxOffice"`
}

GetMovie function reads data from MongoDB and returns JSON

func (db *DB) GetMovie(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    w.WriteHeader(http.StatusOK)
    var movie Movie
    err := db.collection.Find(bson.M{"_id": bson.ObjectIdHex(vars["id"])}).One(&movie)
    if err != nil {
        w.Write([]byte(err.Error()))
    } else {
        w.Header().Set("Content-Type", "application/json")
        response, _ := json.Marshal(movie)
        w.Write(response)
    }

}

I do not understand how generic map bson.M was created. Why did the author used bson.ObjectIdHex(vars["id"]?

icza · Accepted Answer · 2019-04-05T09:57:40.000

bson.M is a map under the hood:

type M map[string]interface{}

And this:

bson.M{"_id": bson.ObjectIdHex(vars["id"])}

Is a composite literal creating a value of type bson.M. It has a single pair where key is "_id" and the associated value is a bson.ObjectId returned by the function bson.ObjectIdHex().

The document ID to look up and return is most likely coming as a hexadecimal string in vars["id"], and bson.ObjectIdHex() converts (parses) this into an ObjectId.

Tips: to query a document by ID, easier is to use Collection.FindId, e.g.:

err := db.collection.FindId(bson.ObjectIdHex(vars["id"])).One(&movie)

Also to avoid a runtime panic in case an invalid ID is stored in vars["id"], you could use bson.IsObjectIdHex() to check it first. For details, see Prevent runtime panic in bson.ObjectIdHex.

Also, marshaling the result into a byte slice and then writing it to the response is inefficient, the response could be streamed to the output using json.Encoder. For details, see Ouput json to http.ResponseWriter with template.

How Find function's generic map was created?

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