<div class="field">
  <%= f.label :delivery_areas %>
    <% Area.all.each do |area| %>
      <%= check_box_tag("area", "name") %>
      <%= area.name %>
    <% end %>
 </div>

def index
  @restaurants = Restaurant.all.order("created_at DESC")
  if params[:area].blank?
    @restaurants = Restaurant.all.order("created_at DESC")
  else
    @area_id = Area.find_by(name: params[:area]).id
    @restaurants = Restaurant.where(:area_id => @area_id).order("created_at DESC")
  end
  if @reviews.blank?
      @avg_review = 0
    else
      @avg_review = @reviews.average(:rating).round(2)
    end
end

<ul>
      <li class="dropdown area-dropdown">
        <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true" style="width: 440px;">
         area
          <span class="caret"></span>
        </button>

        <ul class="dropdown-menu scrollable-menu" aria-labelledby="dropdownMenu1" style="width: 440px;">
          <% @areas.each do |area| %>
            <li>
              <%= link_to area.name, restaurants_path(area: area.name), class: "link" %>
            </li>
          <% end %>
        </ul>
      </li>
    </ul>