Findone() always return blank with mongodb driver in golang-CodePudding

I'm trying to search a document in golang using mongodb driver. But the result is always blank.

This is my code:

var bsonString bson.M
    var jsonString string
    fmt.Printf("[%s] > request for url\n", req.RemoteAddr)
    w.Header().Set("Access-Control-Allow-Origin", "*")
    dataSource := client.Database(dbName)
    collection := dataSource.Collection(collectionName)
    err := collection.FindOne(context.Background(), bson.D{{"question.title", "Question"}}).Decode(&bsonString)
    if err != nil {
        if err == mongo.ErrNoDocuments {
            // This error means your query did not match any documents.
            log.Println("No matched documents!")
            return
        }
        panic(err)
    }
    finalBytes, _ := bson.Marshal(bsonString)
    bson.Unmarshal(finalBytes, &jsonString)
    fmt.Println(jsonString)

My data is:

{"_id":{"$oid":"631c5c78e606582e2ad78e2d"},"question":{"title":"Question","create_by":"AZ","time":{"$numberLong":"1661394765044"},"detail":"<h4>info</h4>"},"answers":[{"create_by":"baa","time":{"$numberLong":"1661394765044"},"detail":"<h4>abc</h4>"}]}

CodePudding user response：

You are unmarshalling a document into a string. If you did error handling you would see the error:

// no error haldling
json.Unmarshal(finalBytes, &jsonString)

// with error handling
err = json.Unmarshal(finalBytes, &jsonString)
if err != nil {
    fmt.Println(err)
    // prints: json: cannot unmarshal object into Go value of type string
}

You can create a struct to match your data or unmarshal into interface (for unstructured data).

// declare variables
var bsonString bson.M
var jsonString string
dataSource := client.Database(dbName)
collection := dataSource.Collection(collectionName)
//
dataObject := map[string]interface{}{}

fmt.Printf("[%s] > request for url\n", req.RemoteAddr)

// set headers
w.Header().Set("Access-Control-Allow-Origin", "*")


err := collection.FindOne(context.Background(), bson.D{{"question.title", "Question"}}).Decode(&bsonString)
if err != nil {
    if err == mongo.ErrNoDocuments {
        // This error means your query did not match any documents.
        log.Println("No matched documents!")
        return
    }
    panic(err)
}

// marshal & unmarshall
finalBytes, _ := bson.Marshal(bsonString)
bson.Unmarshal(finalBytes, &dataObject)
fmt.Println(dataObject)