Using MongoDB Projection - mongodb

Question

I have the following structure in my database:
{
"_id": {
"$oid": "5fc4fc68fcd604bac9f61f71"
},
"init": {
"fullname": "Besikta Anibula",
"parts": [
"Besikta",
"Anibula"
],
"alt": "Besikta Ani."
},
"industry": "E-Commerce"
}
I´m trying to just access the init object and write the results to a structured variable in Go:
var InputData struct {
Fullname string `bson:"fullname" json:"fullname"`
Parts []string`bson:"parts" json:"parts"`
Alt string `bson:"alt" json:"alt"`
}
collectionRESULTS.FindOne(ctx, options.FindOne().SetProjection(bson.M{"init": 1})).Decode(&InputData)
js, _ := json.Marshal(InputData)
fmt.Fprint(writer, string(js))
But the result is empty:
{"fullname":"","parts":null,"alt":""}
It is working when not using a projection like:
var InputData struct {
Ident primitive.ObjectID `bson:"_id" json:"id"`
}
collectionRESULTS.FindOne(ctx, bson.M{}).Decode(&InputData)
js, _ := json.Marshal(InputData)
fmt.Fprint(writer, string(js))
Result as expected:
{"id":"5fc4fc68fcd604bac9f61f71"}

Answer 1

You set the projection to only retrieve the init property of the result documents, and then you try to unmarshal into a struct value that does not have any matching field for the init value, so nothing will be set in that struct value.
You must use a value that has an init field, like this wrapper Result struct:
type Result struct {
Init InputData `bson:"init"`
}
type InputData struct {
Fullname string `bson:"fullname" json:"fullname"`
Parts []string `bson:"parts" json:"parts"`
Alt string `bson:"alt" json:"alt"`
}
Use it like this:
var result Result
err := collectionRESULTS.FindOne(ctx, bson.M{}, options.FindOne().
SetProjection(bson.M{"init": 1})).Decode(&result)
if err != nil {
// handle error
}