How to extract fields from mongodb record when iterating with cursor in Golang

Question

I am fairly new to golang programming and the mongodb interface.

I've got a dbase of records created by another application. I am trying to walk the dbase and examine specific fields of each record. I can decode the full records as bson, but I cannot get the specific values.

This struct defines the 3 fields I would like to extract:

type myDbaseRec struct {
    aid        string  `bson:"pon-util-aid"`
    ingressPct string  `bson:"ingress-bucket-percent"`
    egressPct  string  `bson:"egress-bucket-percent"`
}

Here is my code to iterate through the cursor after the collection.Find(ctx, queryFilter) and decode the results both as bson and as my struct:

    var myResult myDbaseRec
    var bsonMResult bson.M

    var count int
    for cursor.Next(ctx) {

        err := cursor.Decode(&myResult)
        if err != nil {
            fmt.Println("cursor.Next() error:", err)
            panic(err)
            // If there are no cursor.Decode errors
        } else {
            fmt.Println("\nresult type:", reflect.TypeOf(myResult))
            fmt.Printf("result: %+v\n", myResult)

        }

        err = cursor.Decode(&bsonMResult)
        if err != nil {
            fmt.Println("bson decode error:", err)
            panic(err)
            // If there are no cursor.Decode errors
        } else {
            fmt.Println("\nresult type:", reflect.TypeOf(bsonMResult))
            fmt.Println("\nresult:", bsonMResult)
        }

    }

Here is an example of one iteration of the loop. The bson decode appears to work, but my struct is empty:

result type: internal.myDbaseRec
result: {aid: ingressPct: egressPct:}

result type: primitive.M

result: map[pon-util-aid:ROLT-1-MONTREAL/1/1/xp2 _id:ObjectID("5d70b4d1b3605301ef72228b") 
admitted-assured-upstream-bw:0 admitted-excess-upstream-bw:0 admitted-fixed-upstream-bw:0 
assured-upstream-bytes:0 available-excess-upstream-bw:0 available-fixed-upstream-bw:622080 
app_counters_key_field:ROLT-1-MONTREAL/1/1/xp2 app_export_time:1567665626 downstream-octets:52639862633214 
egress-bucket-bps:8940390198 egress-bucket-percent:91 egress-bucket-seconds:559 
excess-upstream-bytes:0 fixed-upstream-bytes:0 ingress-bucket-bps:8253153852 
ingress-bucket-percent:84 ingress-bucket-seconds:559 sample-time:0 upstream-octets:48549268162714]

I would have expected to get

result: {aid:"ROLT-1-MONTREAL/1/1/xp2" ingressPct:84 egressPct:91}

Any suggestion on how to properly find these 3 fields from each record?

=== UPDATE: The first comment below answered my question.

Wan B. · Accepted Answer · 2019-09-06 23:56:46Z

1

First, in Go only fields starting with a (Unicode) upper case letter are exported. See also Exported identifiers. The default decoder will try to decode only to the exported fields. So you should change the struct into:

type myDbaseRec struct {
    Aid        string  `bson:"pon-util-aid"`
    IngressPct int32  `bson:"ingress-bucket-percent"`
    EgressPct  int32  `bson:"egress-bucket-percent"`
}

Also notice that the struct above for IngressPct and EgressPct have type int32. This is because the value in the document is represented in numbers (int/double), and not string. You may change it to other number type accordingly i.e. int16, int64, etc.

answered Sep 6, 2019 at 23:56

Wan B.

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1 Comment

Steve Simon Over a year ago

Thank you very much! Changing the struct the way you suggest allows me to get the values from the database records now! This was a big help!

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