Is there a special way that you should call an object when you want to add a where clause that relates to a relationship that has been defined in the model.

An example would be that I have an image, the image belongs\_to or has\_many (whatever rocks your boat) a category, and I want to select all images that don't have any associated categories. 

So for a simple belongs\_to I could just say:

```
Image.where('category_id is null')

```

But is there a better way of doing this since the relationship has been explicitly defined in the model?