This code break Datagrid internals abstraction: `Core` should never know about `Columns`.

It will be better to move adding of the columns to the `Datagrid::Columns` module and plug it by extending `assets` method like:

``` ruby 
def assets
   driver.append_column_queries(super)
end
```

`append_column_queries` supposed to be new driver method that makes magic for AR and raises NotImplementedError with proper message for other ORMs.