Mocking Redis Constructor with Sinon

I'm trying to figure out a way to mock redis in this module:

const Redis  = require('ioredis');
  const myFunction = {
    exists: (thingToCheck) {
      let redis_client = new Redis(
        6379,
        process.env.REDIS_URL,
        {
          connectTimeout: 75,
          dropBufferSupport: true,
          retryStrategy: functionHere
        });

    redis_client.exists(thingToCheck, function (err, resp) {
     // handlings in here
    });
  }
};

Using this test-code:

const LambdaTester = require('lambda-tester');
const chai = require('chai');
const expect = chai.expect;
const sinon = require('sinon');
const mockRedis = sinon.mock(require('ioredis'));

describe( 'A Redis Connection error', function() {
    before(() => {
        mockRedis.expects('constructor').returns({
            exists: (sha, callback) => {
                callback('error!', null);
            }
        });
      });

      it( 'It returns a database error', function() {
          return LambdaTester(lambdaToTest)
              .event(thingToCheck)
              .expectError((err) => {
                   expect(err.message).to.equal('Database error');
              });
      });
});

I also tried a few variations, but I'm kind of stuck as I basically need to mock the constructor and I'm not sure Sinon supports this?

mockRedis.expects('exists').returns(
  (thing, callback) => {
    callback('error!', null);
  }
);
sinon.stub(mockRedis, 'constructor').callsFake(() => console.log('test!'));
sinon.stub(mockRedis, 'exists').callsFake(() => console.log('test!'));

Not sure what else to try here, I also tried using rewire as suggested here, but using mockRedis.__set__("exists", myMock); never set that private variable.

I want to fake my error paths ultimately.
I would love to hear what others are doing to test redis in node js .