model.spec.js

const MyModel = require('./mymodel.js');

describe('My Model', () => {
  let model;
  
  beforeEach(() => {
    model = new MyModel();  
  });

  describe('hooks', () => {
    describe('pre save', () => {
      beforeEach(() => {
        sinon.spy(modelUtils, 'myMethod');
      });

      afterEach(() => {
        modelUtils.myMethod.restore();
      });

      it('should use modelUtils.myMethod', (done) => {
        model.save((err) => {
          try {
            expect(modelUtils.myMethod).to.have.been.called;
          } catch (assertErr) {
            err = err || assertErr;
          }

          done(err);
        });
      });
    });
  });
});

mymodel.js

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const utils = require('./utils');

const myModelSchema = new Schema({
  hello: 'world'
});

myModelSchema.pre('save', (next) => {
  utils.myMethod.call(this);
  next();
});

module.exports = mongoose.model('MyModel', myModelSchema);