Rather than `require('validator').check` in every model file, could we do something like this here:

``` javascript
const validationContext = {
  check: require('validator').check
};

...
try {
  const validator = validation[field] || noop;
  validator.call(validationContext, value);
}
...
```

And then use it something like this in the individual model files:

``` javascript
makeValidator({
  id: function (id) {
    if (typeof id == 'undefined') return;
    this.check(id).isInt();
  }
});
```

Just a thought - might be overcomplicating things, but it seems to make sense.