How to formulate nested schema in GraphQL

Question

I am new to GraphQL. As an experiment, I ran the following Node.js Express server:

var express = require('express');
var { graphqlHTTP } = require('express-graphql');
var { buildSchema } = require('graphql');
var schema = buildSchema(`
  type Student {
     sid: ID,
     name: String,
     subjects: [String!],
     address: Location
  }
  type Location{
     unit: String,
     city: String,
     country: String,
     postCode: Int
  }
`);
var root = {   // The root provides a resolver function for each API endpoint
  address: () => ({
    city: ()=>'London'
  }),
};
var app = express();
app.use('/graphql', graphqlHTTP({
  schema: schema,
  rootValue: root,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at http://localhost:4000/graphql');

When I ran the following query on the interface on a browser:

{
  address {
     city
  }
}

I got an error:

{
    "errors": [
    {
      "message": "Query root type must be provided."
    }
  ]
}

What should be the correct way of doing this?

Answer 1

It turned out that I must define the schema 'Query' (and optionally 'Mutation') for the entry point.

var express = require('express');
var { graphqlHTTP } = require('express-graphql');
var { buildSchema } = require('graphql');

var fakeDatabase = [{sid:parseInt(Math.random()*10000),
                     name:'Philip',
                     subjects:['Chemistry', 'Physics', 'Maths'],
                     address:{
                           unit: 'H505',
                           city: 'London',
                           country: 'United Kingdom',
                           postCode: 33100
}}];

// 'Query' and 'Mutation' are special schemas defining the entry points
var schema = buildSchema(`
  type Query {
     name: String     
     sid(year: Int): ID
     subjects: [String]
     address: Location
  }
  type Location{
     unit: String
     city: String
     country: String
     postCode: Int
  }
  type Mutation{
     setName(nn: String): String
  }
`);

// Each field is either a constant or a callback
var root = {
  name: ()=>fakeDatabase[0].name,
  sid: arg => (arg.year+"-"+fakeDatabase[0].sid),
  subjects: fakeDatabase[0].subjects,
  address: () => ({
     city: ()=>fakeDatabase[0].address.city
  }),
  setName: arg => {fakeDatabase[0].name=arg.nn; return arg.nn;}
};

var app = express();
app.use('/graphql', graphqlHTTP({
  schema: schema,
  rootValue: root,
  graphiql: true,
}));
app.listen(4000);
console.log('Running a GraphQL API server at http://localhost:4000/graphql');