MongoDB returns object data as array of key-value pair in Go

I have written following query which returns me records where updated_at date is greater than synced_at date from all records using mongodb library in Golang.

	pipeline := []bson.M{}
	filter := []string{"$updated_at", "$synced_at"}
	pipeline = append(pipeline, bson.M{"$match": bson.M{"$expr": bson.M{"$gte": filter}}})
    opts := options.Aggregate().SetMaxTime(2 * time.Second)
	cursor, err := collection.Aggregate(ctx, pipeline, opts)
    for cursor.Next(context.Background()) {
		records := model.DData{}
		err = cursor.Decode(&records)
    }

The structure of Data is:

type DData struct {
	Name               string   	`json:"name" bson:"name"` 
	Data               interface{}  `json:"data" bson:"data"`
	UpdatedAt          time.Time    `json:"updated_at" bson:"updated_at"`
	SyncedAt           time.Time     `json:"synced_at" bson:"synced_at"`
}

Data in collection is of the form:

{
	"name":"Vallabh",
	"data":{
		"field1":"value1",
		"field2":"value2",
		"field3":"value3",
	},
	"updated_at":2021-08-17T09:43:27Z,
	"synced_at":2021-08-07T09:43:27Z
}

But with above query I am getting data in the form:

{
	"name":"Vallabh",
	"data":[
		{
			"key":"field1",
			"value":"value1"
		},
		{
			"key":"field2",
			"value":"value2"
		},
		{
			"key":"field3",
			"value":"value3"
		}
	}],
	"updated_at":"2021-08-17T09:43:27Z",
	"synced_at":"2021-08-07T09:43:27Z"
}

What am I doing wrong? Its happening only when field type is an interface in struct.

@VallabhLakade I had similar concern like this and tried the below way that helped .

So basically what's the problem is that the mongo-driver defaults to unmarshalling as bson.D for structs of type interface{} where as mgo mgo-driver defaults to bson.M .

So we will have to add the below code while trying to establish connection with mongo-db , SetRegistry() options as clientOpts To map the old mgo behavior, so that mongo-driver defaults to bson.M while unmarshalling structs of type interface{} , and this should not display the values back as key-value pair

	tM := reflect.TypeOf(bson.M{})
	reg := bson.NewRegistryBuilder().RegisterTypeMapEntry(bsontype.EmbeddedDocument, tM).Build()
	clientOpts := options.Client().ApplyURI(SOMEURI).SetAuth(authVal).SetRegistry(reg)
	client, err := mongo.Connect(ctx, clientOpts)

Reference -> https://stackoverflow.com/questions/74418871/mongodb-document-returns-array-of-key-value-pair-in-go-mongo-driver/74423240#74423240

We also faced the same issue once, where we made two structs one using the interface and another using a struct.

1. When you want to update or Insert you can use the interface.
2. When you want to fetch from database you have to create another structure which using struct as the type for the field.

Try with this

type DData struct {
    Name               string             `json:"name" bson:"name"` 
    Data               map[string]string  `json:"data" bson:"data"`
    UpdatedAt          time.Time          `json:"updated_at" bson:"updated_at"`
    SyncedAt           time.Time          `json:"synced_at" bson:"synced_at"`
}

Always use the type if you know it beforehand, with interface{} the burden is on the library to find out what the type is. If you expect the varied values in the map you can use map[string]interface{}

Found a very weird solution to this problem where I decoded it to bson.M{}, then marshalled it and unmarshalled it to my struct. I am sure there are better ways but this one worked for me.

for cursor.Next(context.Background()) {
		tempResult := bson.M{}
		err = decode(cursor, &tempResult)
		if err != nil {
			logger.LogErrorf(err, "error while decoding")
			continue
		}
		obj, err := marshal(tempResult)
		if err != nil {
			logger.LogErrorf(err, "error while marshalling")
			continue
		}
		var data model.DData
		err = json.Unmarshal(obj, &data)
		if err != nil {
			tenant.LogError(err, "error while marshalling")
			continue
		}
}

@Vallabh you shall use mgocompat too something like below this should mimic the old mgo behavior

registry := mgocompat.NewRegistryBuilder().Build()
connectOpts := options.Client().ApplyURI(SOMEURI).SetAuth(info).SetRegistry(registry)