Sugestion: use call or apply, so the programmer do not need to really receive arguments. Only when it wants.

```javascript
this.options.transformFixtureDataFn.apply(data, [data, Model]);
```

So, the use of it can be something like:

```javascript
sequelize_fixtures.loadFile('server/fixtures/*.json', models, {
  transformFixtureDataFn: function () {
    // Fixtures with negative numbers allow creating data objects
    // relative to the time of the import.
    if(data.createdAt && data.createdAt < 0) { 
      this.createdAt = new Date((new Date()).getTime() + parseFloat(data.createdAt) * 1000 * 60);
    }
    // do not need to return ;)
  }
});
```

I suggest too pass Model as first argument so...

```javascript
// ...
  transformFixtureDataFn: function (Model) {
    if (Model.tableName === 'People') {
      // do some stuffs
    }
  }
// ...
```