How to Decode the returns of Aggregate() - mongodb

Question

I'm trying to do a query in MongoDB, just like the group by in SQL. So I use the Aggregate() API, to find all the unique _id(MarketType+Symbol).
And the Aggregate() runs successful, but when I decode the return values, I got nothing. Aggregate() return type is the same as Find(), and I decode in this way when Find() is useful. Why it can't work when I Aggregate().
type MarketSymbol struct {
MarketType string `json:"Market,omitempty" bson:"MarketType"`
Symbol string `json:"Symbol,omitempty" bson:"Symbol"`
}
func GetSymbol() {
pipeline := bson.M {
"$group" :
bson.M{"_id":bson.M{"MarketType":"$MarketType","Symbol":"$Symbol"}},
}
cur,err := mongoSt.Collection.Aggregate(context.TODO(), []bson.M{pipeline})
if err != nil {
fmt.Println("Aggregate Error:", err)
} else {
fmt.Println("Aggregate Success!") //this line works
}
var results []MarketSymbol
count := 1
for cur.Next(context.TODO()) {
var elem MarketSymbol
err := cur.Decode(&elem)
if err != nil {
fmt.Printf("[count]=%d Decode Error\n", count)
count++
continue
}
fmt.Printf("[count]=%d, [MarketType]:%s , [Symbol]:%s\n", count, elem.MarketType, elem.Symbol) //this line works,but value is nil
count++
results = append(results, elem)
}
}

Answer 1

I got it. I also tried to use _id to save the Decode value before, but failed. So I thought it doesn't work. But it's right, what actually caused the error is I gave a wrong type of MarketType, it should be int32 but I gave it a string. So...
Anyway, the problem solved, here is the code.
type ID struct {
Id MarketSymbol `json:"ID,omitempty" bson:"_id"`
}
type MarketSymbol struct {
MarketType int32 `json:"Market,omitempty" bson:"MarketType"`
Symbol string `json:"Symbol,omitempty" bson:"Symbol"`
}
func GetSymbol{
pipeline := bson.M {
"$group" : bson.M{"_id":bson.M{"MarketType":"$MarketType","Symbol":"$Symbol"}},
}
cur,err := mongoSt.Collection.Aggregate(context.TODO(), []bson.M{pipeline})
if err != nil {
fmt.Println("Aggregate Error:", err)
} else {
fmt.Println("Aggregate Success!")
}
var results []ID
count := 1
for cur.Next(context.TODO()) {
var elem ID
err := cur.Decode(&elem)
if err != nil {
fmt.Printf("[count]=%d Decode Error\n", count)
count++
continue
}
fmt.Printf("[count]=%d, [MarketType]:%d , [Symbol]:%s\n", count, elem.Id.MarketType, elem.Id.Symbol)
count++
results = append(results, elem)
}
}