how to mock graphql context in a test

schema.spec.js

// in src/schema.spec.js
const { graphql } = require('graphql');
const schema = require('./schema');

it('responds to the Tweets query', () => {
    // stubs
    const queryStub = q => {
        if (q == 'SELECT * from tweets') {
            return Promise.resolve({ rows: [
                { id: 1, body: 'Lorem Ipsum', date: new Date(), author_id: 10 },
                { id: 2, body: 'Sic dolor amet', date: new Date(), author_id: 11 }
            ]});
        }
    };
    const dataloaders = {
        userById: {
            load: id => {
                if (id == 10 ) {
                    return Promise.resolve({ id: 10, username: 'johndoe', first_name: 'John', last_name: 'Doe', avatar_url: 'acme.com/avatars/10' });
                }
                if (id == 11 ) {
                    return Promise.resolve({
                        { id: 11, username: 'janedoe', first_name: 'Jane', last_name: 'Doe', avatar_url: 'acme.com/avatars/11' });
                }
            }
        }
    };
    const context = { pgClient: { query: queryStub }, dataloaders };
    // now onto the test itself
    const query = '{ Tweets { id body Author { username } }}';
    return graphql(schema, query, null, context).then(results => {
        expect(results).toEqual({
            data: {
                Tweets: [
                    { id: '1', body: 'hello', Author: { username: 'johndoe' } },
                    { id: '2', body: 'world', Author: { username: 'janedoe' } },
                ],
            },
        });
    });
})