using :counter_cache with some conditions

Question

I have the following model:

class EntityTag < ActiveRecord::Base
  attr_protected :user_id, :post_id, :entity_id

  belongs_to :user
  belongs_to :post
  belongs_to :entity

  validates :user_id, :presence => true
  validates :entity_id, :presence => true
  validates :post_id, :presence => true

  validates_uniqueness_of :user_id, :scope => [:entity_id, :post_id]
end

It is leveraged in the Post model as follows:

class Post < ActiveRecord::Base
  ...
  has_many :entity_tags, :dependent => :destroy
  has_many :tags, :through => :entity_tags, :uniq => true,
    :source => :entity
  has_many :taggers, :through => :entity_tags, :uniq => true,
    :source => :user
  ...
end

I want keep counts for two things: 1) The number of entity_tags for a particular post_id. This can be done via :counter_cache on the :post in the EntityTag model. 2) I also want to keep track of the number of UNIQUE EntityTags based on unique :entity_id values for a particular post_id. How do I go about achieving this? Can I use :counter_cache with some sort of constraint? Or is there a completely different approach?