Good notes posted by harm

This is ON by default in :has_many

When defining a has_many relationship this behaviour is on by default. See has_many documentation, look for the :validate flag.

Adding params to generated url

Whenever you want to append custom parameters to a to be generated url it might be necessary to stop url_for from escaping.

url_for(:action => 'some_action', :custom1 => 'some_value', :custom2 => 'some_value', :escape => false)

If the escape => false option is not passed the generated url contains & instead of the correct &-sign.