error=topology is closed

[Kai Hendry]

Hi, I'm struggling to get your example from https://godoc.org/github.com/mongodb/mongo-go-driver/mongo

working on go version go1.10.2 linux/amd64. What am I missing? Tbh I don't quite understand all those context.Background() stuff, but I am going along with it.

[hendry@t480s lambda2mongo]$ git --git-dir /home/hendry/go/src/github.com/mongodb/mongo-go-driver/.git describe                                                

v0.0.5-24-gcf7d3d6                     

[hendry@t480s lambda2mongo]$ cat main.go                                       

package main                           

import (                               

        "context"                      

        "github.com/apex/log"          

        "github.com/mongodb/mongo-go-driver/bson"                              

        "github.com/mongodb/mongo-go-driver/mongo"                             

)                                      

func main() {                          

        client, err := mongo.NewClient("mongodb://localhost:27017")            

        if err != nil {                

                log.WithError(err).Fatal("error connecting")                   

        }                              

        collection := client.Database("admin").Collection("system.version")    

        cur, err := collection.Find(context.Background(), nil)                 

        if err != nil {                

                log.WithError(err).Fatal("failed to find collection")          

        }                              

        defer cur.Close(context.Background())                                  

        for cur.Next(context.Background()) {                                   

                elem := bson.NewDocument()                                     

                err := cur.Decode(elem)                                        

                if err != nil {        

                        log.WithError(err).Fatal("failed to decode element")   

                }                      

                // do something with elem....                                  

        }                              

        if err := cur.Err(); err != nil {                                      

                log.WithError(err).Fatal("failed to get cursor")               

        }                              

}                                      

[hendry@t480s lambda2mongo]$ go run main.go                                    

2018/05/28 15:32:18 fatal failed to find collection error=topology is closed   

exit status 1                          

[hendry@t480s lambda2mongo]$ mongo mongodb://localhost:27017                   

MongoDB shell version v3.6.4           

connecting to: mongodb://localhost:27017                                       

MongoDB server version: 3.6.4          

Server has startup warnings:           

2018-05-28T14:37:21.072+0800 I STORAGE  [initandlisten]                        

2018-05-28T14:37:21.072+0800 I STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine        

2018-05-28T14:37:21.072+0800 I STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem                                    

2018-05-28T14:37:21.580+0800 I CONTROL  [initandlisten]                        

2018-05-28T14:37:21.580+0800 I CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.                                            

2018-05-28T14:37:21.580+0800 I CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.                           

2018-05-28T14:37:21.580+0800 I CONTROL  [initandlisten]                        

> use admin                            

switched to db admin                   

> db.system.version.find()             

{ "_id" : "featureCompatibilityVersion", "version" : "3.6" }                   

>                                      

bye                                    

[hendry@t480s lambda2mongo]$           

[Rajiv Subramanian M]

After this lines

client, err := mongo.NewClient("mongodb://localhost:27017")            

if err != nil {

    log.WithError(err).Fatal("error connecting")

}                              

Include

client.Connect(context.Background())

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Rajiv

[Kai Hendry]

Thanks Rajiv, it now doesn't error out.

But how do I get some output from this cursor { "_id" : "featureCompatibilityVersion", "version" : "3.6" } ?

Are there some complete examples somewhere I could study?               

[Rajiv Subramanian M]

Yes there are examples here: https://github.com/mongodb/mongo-go-driver/tree/master/examples/documentation_examples

To query on collection you do this,

cur, err := collection.Find(

   context.Background(),

   bson.NewDocument(bson.EC.String("_id", "featureCompatibilityVersion"), bson.EC.String("version", "3.6")),

)

[Kai Hendry]

It's not clear how to map JSON to BSON. Are there some examples to do this?

My use case is that I need to update a MongoDB with JSON payloads.

Many thanks,

[Rajiv Subramanian M]

The bson library used in mongo-go-driver is their own implementation of bson. Not sure if any package available do convert json to bson but found this one on stackoverflow https://stackoverflow.com/questions/39785289/how-to-marshal-json-string-to-bson-document-in-golang-for-writing-to-mongodb?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa