def index
  @products = product.all

  if params[:country]
    @products = Country.find(params[:country]).products
  end

  if params[:category]
    @products = Category.find(params[:category]).products
  end

end

class Product < ActiveRecord::Base

  has_many :categorization
  belongs_to :country

end

class Category < ActiveRecord::Base
  has_many :categorization
  has_many :products, through: :categorization

  attr_accessible :name

end

class Categorization < ActiveRecord::Base
  belongs_to :category
  belongs_to :product

  attr_accessible :category, :product
end

<div class="wrapper">
  <div class="products">
  </div>
  <div class="sidebar>
   <h2>Categories</h2>
   <ul>
     <% @categories.each do |category| %>
       <li><%= link_to category.name, params.merge(category: category.id) %></li>
     <% end %>
   </ul>
    <h2>Countries</h2>
   <ul>
     <% @countries.each do |country| %>
       <li><%= link_to country.name, params.merge(country: country.id) %></li>
     <% end %>
   </ul>
  </div>
</div>

# Product Model

class Product < ActiveRecord::Base

  # Search filter
  def self.filter_by_params(params)
    if params.has_key?(:category) && params.has_key?(:country_id)
      scoped.joins(:categorization).where("category_id = ? AND country_id = ?", params[:category], params[:country_id])
    elsif params.has_key?(:category)
      scoped.joins(:categorization).where("category_id = ?", params[:category])
    elsif params.has_key?(:country_id)
      scoped.where(country_id: params[:country_id])
    else
      scoped
    end
  end

end

# Product Controller

def index
  @product = Product.filter_by_params(params)
end

def index
  @products = Product.filter_by_params(params)
end

def self.filter_by_params(params)
  scoped = self.scoped.where(:country_id => params[:country_id]) if params[:country_id]
  scoped = scoped.where(:category_id => params[:category_id]) if params[:category_id]
  scoped
end