Mongoose document: Execute hook manually for testing

I'd like to test some document transformation which is executed in a Mongoose pre save hook. Simplified example:

mySchema.pre('save', function(callback) {
  this.property = this.property + '_modified';
  callback();
});

Test:

var testDoc = new MyDocument({ property: 'foo' });
// TODO -- how to execute the hook?
expect(testDoc.property).to.eql('foo_modified');

How can I manually execute this hook?