Bind model context to lifecycle callback

I wanted to set a "counter" on my model depending on a criteria.
```javascript
beforeValidate: function(values, cb) {
  this.findOne({
    where: criteria,
    sort: { updatedAt: 'desc' }
  }).exec(function(err, user) {
    values.counter = user ? 0 : user.counter++;
  });
```
Currently the context passed is the model prototype, why not the model ?
I could access to findOne but I got an error because _transformer is undefined.