Form to create entries in many-to-many relationshi

My application consists of these five models: A supermarket can have different categories of products and the products in these categories can be produced by several brands.

Now I want to have one (or two) Selection-field(s) in my supermarket-form in which I can select one element in Category with its name appearing and one or more element(s) in Brand with its name appearing, so this could be stored in Origin.

I think I could use collection_select, but how do I utilize it here?

class Supermarket < ActiveRecord::Base
  has_many :supplies
  has_many :origins, :through => :supplies
end

class Supply < ActiveRecord::Base  
  belongs_to :origin  
  belongs_to :supermarket  
end

class Origin < ActiveRecord::Base
  belongs_to :category
  belongs_to :brand
end

class Category < ActiveRecord::Base
  has_many :origins
end

class Brand < ActiveRecord::Base
  has_many :origins
end

Probably, I also have to tweak the models...

Edit

To clarify what the outcome of the form should be:

In the form to edit Supermarkets I want to select a Category of products and the corresponding Brands so that I know, which Category and which Brands in this Category is/are sold in this specific Supermarket:

Supermarket (Form):

Name of Supermarket: Walmart

Category (Select one):

Cola (Category_ID 1)
Cornflakes (Category_ID 2)
...

Brand (multiple Select)

The Coca-Cola Company (Brand_ID 1)
PepsiCo (Brand_ID 2)
Kellogg Company (Brand_ID 3)
...

This should create entries in Origin like:

Supermarket_ID Category_ID Brand_ID 
      1            1          1 
      1            1          2 
      2            1          2 
      2            2          3 
     ...          ...        ...

/Edit

Edit 2

According to the answer to this question, I could create or select an entry via console like this:

walmart = Supermarket.create(:name => "Walmart");
cornflakes = Category.create(:name => "Corn Flakes");
kellogs = Brand.create(:name => "Kellog's");

walmart.origins.create(:category_id => cornflakes, :brand_id = kellogs)

How can I make use of this via a form? How could I utilize select or collection_select or is there even another helper I could use?

/Edit 2

I have also had a similar problem when utilizing many-to-many relationships. One worthwhile gem to check out would be simple_form.

They have a nice form helper for to_many associations.

In terms of actually implementing the form, I'm not sure I understood how the form data will be used. Regardless, javascript and virtual attributes are your friends here.

EDIT:

Ah, I see what you're saying. the way I would do this would be to create one select_field, for selecting the category, then I would create another select_field (setting :multiple => true) so I can select multiple brands.

<%= select_tag "supermarket[supplies][][category]" ... %>
<%= select_tag "supermarket[supplies][][brands]", ... %> <!-- be sure to set multiple to true !-->

Next, you need to associate everything together. So for that, I would create a virtual attribute that handles the form submission. For example:

def supplies=(supplies_hash)
# read supplies_hash and create the active records. 
end

def supplies

end

Finally, if you're insisting on doing this without a gem, I recommend you take a look at the nested form railscast. to get an idea of what's involved.