Expected variables json to be an object

Question

I am trying to create a graphql server with mongodb. the server is up and running but the only problem I am facing is, I am not able to run my queries.every time i try to run a query it gives me the following error:"Expected variables json to be an object" mutations are execution fine.

when a run a mutation to create a new contact, the document gets added in the mongodb collection I think the mongodb objectId is not getting converted to the compatible type

index.js
const express = require('express')
const {ApolloServer,gql } = require('apollo-server-express')
const app = express()
const typeDefs = require('./typedefs');
const resolvers = require('./resolvers');
const mongoose = require('mongoose')



const startServer = async ()=>{
    const apolloServer = new ApolloServer({
        typeDefs,
        resolvers
    });

    await apolloServer.start();

    apolloServer.applyMiddleware({app:app});
    app.use((req,res)=>{
        res.send('express server');
    });

    await mongoose.connect('mongodb://localhost:27017/contact_db',{
        useUnifiedTopology:true,
        useNewUrlParser:true,
    });
    console.log("mongoose connected")

    app.listen(4000,()=>{console.log("server running on port 4000")})
}
startServer();


    typeDefs.js
const {gql} = require('apollo-server-express')


const typeDefs = gql`
type Contact {
    _id:ID!
    firstName:String!
    lastName:String!
    phone:String!
}

input ContactInput {
    firstName: String!
    lastName:String!
    phone:String!
}

type Query {
    getAllContacts:[Contact!]!
    getContact(id:ID!):Contact
}
type Mutation {
    createContact(contact:ContactInput): Contact
}
`
module.exports = typeDefs

    resolver.js
const Contact = require('./modals/contact.model')

const prepare = (o) => {
    o._id = o._id.toString()
    return o
  }

const resolvers = {
    Query:{
        getAllContacts:async ()=>{
            return (await Contact.find({}).toArray()).map(prepare)
        },
        getContact:async (parent,{id},context,info)=>{
                return await Contact.findById(id);
        }
    },
    Mutation:{
        createContact: async (parent,args,context,info)=>{
            const {firstName,lastName,phone} = args.contact;
            const contact = new Contact({firstName,lastName,phone});
            await contact.save();
            return contact;
        }
    }
}
module.exports = resolvers

    contect.model.js
const mongoose  = require('mongoose')

const contactSchema = new mongoose.Schema({
    _id: {
        type:mongoose.Schema.Types.ObjectId
    },
    firstName:{
        type:String,
        required:true
    },
    lastName:{
        type:String,
        required:true
    },
    phone:{
        type:String,
        required:true
    }
})

const Contact = mongoose.model('contact',contactSchema);
module.exports = Contact;

Answer 1

I had this problem when running locally and using the Apollo playground/studio.

The problem is the variable I was trying to pass into the query from the playground.

I was passing a single int "1" when I needed to pass in an object "{ "weightId": "1" }"

Look for the Variables box underneath the Operation box in the playground. Hope this helps out anyone else who's stuck!