Sort on fields from other column. Receive ActiveRecord::StatementInvalid

gistfile1.txt

Error Log:
,,,ruby
2011-06-26T01:25:07+00:00 app[web.1]: Started GET "/admin/problems?sort=user_address&sort_reverse=false" for 76.114.20.226 at 2011-06-25 18:25:07 -0700
2011-06-26T01:25:07+00:00 app[web.1]: 
2011-06-26T01:25:07+00:00 app[web.1]: ActiveRecord::StatementInvalid (PGError: ERROR:  syntax error at or near "{"
2011-06-26T01:25:07+00:00 app[web.1]: LINE 1: ...T  "problems".* FROM "problems" ORDER BY problems.{User(id: ...
2011-06-26T01:25:07+00:00 app[web.1]:                                                              ^
2011-06-26T01:25:07+00:00 app[web.1]: : SELECT  "problems".* FROM "problems" ORDER BY problems.{User(id: integer, email: string, encrypted_password: string, reset_password_token: string, reset_password_sent_at: datetime, remember_created_at: datetime, sign_in_count: integer, current_sign_in_at: datetime, last_sign_in_at: datetime, current_sign_in_ip: string, last_sign_in_ip: string, created_at: datetime, updated_at: datetime, first_name: string, last_name: string, location: string, address: string, postal_code: string, city: string, state: string, country: string, latitude: float, longitude: float, account_status: string)=>:address} asc LIMIT 20 OFFSET 0):
```

Rails Admin initalizer
```ruby
  config.model Problem do
    list do
      field :user_first_name, :string do
        formatted_value do
          bindings[:object].user.try :first_name
        end
        sortable do
          { User => :first_name }
        end
        searchable do
          { User => :first_name }
        end
      end
    end
  end
```