class Active < EZ::Where::Compositions::Base
    def prepare(klass)
      active  == true  if klass.column_names.include? 'active'
      visible == true  if klass.column_names.include? 'visible'
      hidden  == false if klass.column_names.include? 'hidden'
      deactivated_at == :null if klass.column_names.include? 'deactivated_at'
    end
  end

  class Published < EZ::Where::Compositions::Base
    def prepare(klass, now = Time.new)
      append Active.new(klass)
      date_cond = create_condition
      date_cond += c { publish_at == :null; unpublish_at == :null }
      date_cond |= c { publish_at <= now } + (c { unpublish_at > now } | c { unpublish_at == :null })
      append date_cond
    end
  end


Article.composition(:active).to_sql

=> ["articles.active = ? AND articles.hidden = ?
       AND articles.deactivated_at IS NULL", true, false]


Article.composition(:published, Time.new.beginning_of_day).to_sql

=>["(articles.active = ? AND articles.hidden = ? AND
      articles.deactivated_at IS NULL) AND
      ((articles.publish_at IS NULL AND articles.unpublish_at IS NULL) OR
     ((articles.publish_at <= ?) AND ((articles.unpublish_at > ?) OR
     (articles.unpublish_at IS NULL))))", true, false,
     Time.new.beginning_of_day,  Time.new.beginning_of_day]