Constructing Types

For many apps, you can define a fixed schema when the application starts, and define it using GraphQL schema language. In some cases, it’s useful to construct a schema programmatically. You can do this using the GraphQLSchema constructor.

When you are using the GraphQLSchema constructor to create a schema, instead of defining Query and Mutation types solely using schema language, you create them as separate object types.

For example, let’s say we are building a simple API that lets you fetch user data for a few hardcoded users based on an id. Using buildSchema we could write a server with:

const express = require('express');
const { createHandler } = require('graphql-http/lib/use/express');
const { buildSchema } = require('graphql');
 
const schema = buildSchema(`
  type User {
    id: String
    name: String
  }
 
  type Query {
    user(id: String): User
  }
`);
 
// Maps id to User object
const fakeDatabase = {
  a: {
    id: 'a',
    name: 'alice',
  },
  b: {
    id: 'b',
    name: 'bob',
  },
};
 
const root = {
  user({ id }) {
    return fakeDatabase[id];
  },
};
 
const app = express();
app.all(
  '/graphql',
  createHandler({
    schema: schema,
    rootValue: root,
  }),
);
app.listen(4000);
console.log('Running a GraphQL API server at localhost:4000/graphql');

When we use the GraphQLSchema constructor method of creating the API, the root level resolvers are implemented on the Query and Mutation types rather than on a root object.

This can be particularly useful if you want to create a GraphQL schema automatically from something else, like a database schema. You might have a common format for something like creating and updating database records. This is also useful for implementing features like union types which don’t map cleanly to ES6 classes and schema language.

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