ruby on rails - How to generate array for grouped_options_for_select based on models -

a product belogs_to category. category has_many products

i need select tag includes optgroup tag.

so basically, need html markup similar pseudo code:

<select name="product" class="products">   <option value=""></option>   <optgroup label="cheese">     <option value="1">cheese 1</option>     <option value="2">cheese 2</option>     <option value="3">cheese 3</option>   </optgroup>   <optgroup label="juices">     <option value="3">juice 1</option>     <option value="4">juice 2</option>     <option value="5">juice 3</option>   </optgroup> </select> 

based on models, how generate array put grouped_options_for_select? example, best approach generate grouped_options?

<%= select_tag 'product', grouped_options_for_select(grouped_options), :include_blank => true , :id => nil, :class => 'product' %> 

controller:

@products = product.all(:include => :category) @grouped_options = @products.inject({}) |options, product|   (options[product.category.name] ||= []) << [product.name, product.id]   options end 

view:

<%= select_tag 'product', grouped_options_for_select(@grouped_options), :include_blank => true , :id => nil, :class => 'product' %>