Counter cache

Rails
1

class Category
has_many :clips
end

class Clip
belongs_to :category, :counter_cache = > true

named_scope :converted, :conditions => {:converted => true}
end

  1. How to update counters if clip changes its category_id ?
  2. How to implement custom counters by conditions, e.g:

category.clips_count
category.converted_clips_count