trying to get @product_results to not contain products who are already in a given @category

product.rb

class Product < ActiveRecord::Base
  
  
  has_and_belongs_to_many :product_groups

...

  scope :in_categories, lambda {|ids| where(:category_id => ids) }
  scope :visible, where(:visible => true)
  scope :search, lambda {|kw| where(
      ['name LIKE :kw OR keywords LIKE :kw OR number LIKE :kw', :kw => "%#{kw}%"]
  )}
  
 end

product_group.rb

class ProductGroup < ActiveRecord::Base
  
  has_and_belongs_to_many :products
  
  attr_accessible :name
  
  validates :name, :presence => true

end

product_groups_controller.rb

def add_remove
    @product_group = ProductGroup.find(params[:id])
    @products = @product_group.products
    @product_results = Product.search(params[:search])
    if params[:search].nil? || params[:search].empty?
      @product_results = []
    else
      @product_results = Product.search(params[:search]) 
    end
  end