Graphql : Schema is not configured for mutations in Go

I am trying to write Graphql Mutation inside Go. I am using github.com/graphql-go/graphql library.

Here is the code i have written for the same.

package main

import (
  "bytes"
  "encoding/json"
  "fmt"
  "io/ioutil"
  "net/http"

  "github.com/graphql-go/graphql"
)

type userMaster struct {
  Name string `json:"userName"`
  ID   string `json:"emailID"`
}

var user userMaster

func main() {
fmt.Println("Hello world !!!")

userType := graphql.NewObject(
    graphql.ObjectConfig{
        Name: "userMaster",
        Fields: graphql.Fields{
            "userName": &graphql.Field{
                Type: graphql.String,
            },
            "emailID": &graphql.Field{
                Type: graphql.String,
            },
        },
    },
)

rootMutation := graphql.NewObject(graphql.ObjectConfig{
    Name: "Mutations",
    Fields: graphql.Fields{
        "createUser": &graphql.Field{
            Type: userType,
            Args: graphql.FieldConfigArgument{
                "userName": &graphql.ArgumentConfig{
                    Type: graphql.NewNonNull(graphql.String),
                },
                "emailID": &graphql.ArgumentConfig{
                    Type: graphql.NewNonNull(graphql.String),
                },
            },
            Resolve: func(params graphql.ResolveParams) (interface{}, error) {
                user.Name = params.Args["userName"].(string)
                user.ID = params.Args["emailID"].(string)
                return user, nil
            },
        },
    },
})

schema, _ := graphql.NewSchema(graphql.SchemaConfig{
    Mutation: rootMutation,
})

http.HandleFunc("/graphql", func(w http.ResponseWriter, r *http.Request) {
    result := graphql.Do(graphql.Params{
        Schema:        schema,
        RequestString: r.URL.Query().Get("query"),
    })
    json.NewEncoder(w).Encode(result)
})

http.ListenAndServe(":8080", nil)
}

When i am trying to run command from postman I am getting error message {"data":null,"errors":[{"message":"Schema is not configured for mutations","locations":[{"line":1,"column":1}]}]}

http://localhost:8080/graphql?query=mutation+_{createUser(userName:"ABC",emailID:"abc@abc.com"){userName,emailID}}

this is the URL I am trying to hit from postman. I am referring Getting Started With GraphQL Using Golang to implement mutation in Go

Can anyone please help what changes needs to be done here?

The shema you are using for the mutation requires a query object.

schema, _ := graphql.NewSchema(graphql.SchemaConfig{
    // QUERY NEEDED HERE !
    Mutation: rootMutation,
})

A query for userType can be defined like this and added to the schema.

rootQuery := graphql.NewObject(graphql.ObjectConfig{
    Name: "Query",
    Fields: graphql.Fields{
        "lastUser" : &graphql.Field{ // "lastUser" name can be anything
            Type: userType,
        },
    },
})

schema, _ := graphql.NewSchema(graphql.SchemaConfig{
    Query:    rootQuery,
    Mutation: rootMutation,
})

This will fix the "Schema is not configured for mutations" error and have your mutations successfully performed.