"Must provide an operation." error

[masoud-moeini]

I have a simple project like the below one. the database works fine because when I request this endpoint :

http://localhost:8080/api?query={movie(id:1){name}}

I get a proper response. But when I do the same with graphiql endpoint like this :

http://localhost:8080/graphiql?query={movie(id:1){name}}

I get this error :

{
  "data": null,
  "errors": [
    {
      "message": "Must provide an operation.",
      "locations": []
    }
  ]
}

Project:

var movieType = graphql.NewObject(
	graphql.ObjectConfig{
		Name: "Movie",
		Fields: graphql.Fields{
			"id": &graphql.Field{
				Type: graphql.Int,
			},
			"name": &graphql.Field{
				Type: graphql.String,
			},
			"year": &graphql.Field{
				Type: graphql.Int,
			},
			"directorId": &graphql.Field{
				Type: graphql.Int,
			},
			"seriesId": &graphql.Field{
				Type: graphql.Int,
			},
		},
	},
)

var queryType = graphql.NewObject(
	graphql.ObjectConfig{
		Name: "Query",
		Fields: graphql.Fields{
			"movie": &graphql.Field{
				Type: movieType,
				Args: graphql.FieldConfigArgument{
					"id": &graphql.ArgumentConfig{
						Type: graphql.Int,
					},
				},
				Resolve: func(p graphql.ResolveParams) (interface{}, error) {
					id, ok := p.Args["id"].(int)
					if ok {
						for _, movie := range movies {
							if int(movie.ID) == id {
								return movie, nil
							}
						}
					}
					return nil, nil
				},
			},
		},
	})

var schema, _ = graphql.NewSchema(
	graphql.SchemaConfig{
		Query: queryType,
	},
)

func executeQuery(query string, schema graphql.Schema) *graphql.Result {
	result := graphql.Do(graphql.Params{
		Schema:        schema,
		RequestString: query,
	})
	if len(result.Errors) > 0 {
		fmt.Printf("errors: %v", result.Errors)
	}
	return result
}

func main() {

	graphiqlHandler, err := graphiql.NewGraphiqlHandler("/api")
	if err != nil {
		panic(err)
	}

	http.HandleFunc("/api", func(rw http.ResponseWriter, r *http.Request) {
		result := executeQuery(r.URL.Query().Get("query"), schema)
		json.NewEncoder(rw).Encode(result)
	})
	http.Handle("/graphiql", graphiqlHandler)
	http.ListenAndServe(":8080", nil)
}