How to assign values using fields_for with a has_many association in rails 3 -

i have 2 models described below. make when user creates product, have select categories exist in categories table.

tables:

products: id, name

categories: id, name

categories_products: category_id, product_id

class product     has_and_belongs_to_many :categories     accepts_nested_attributes_for :categories end  class category     has_and_belongs_to_many :products end  class productscontroller < applicationcontroller     def new         @product = product.new         @product.categories.build     end      def create         @product = product.new(params[:product])         if @product.save             redirect_to @product, :notice => "successfully created product."         else             render :action => 'new'         end     end end 

views/products/new.haml

= form_for @product |f|     = f.text_field :name     = f.fields_for :categories |cat_form|         = cat_form.collection_select :id, category.all, :id, :name, :prompt => true 

however, fails , gives me: couldn't find category id=3 product id=

i able assign existing category product on creation. there easy way this?

you need use accepts_nested_attributes_for if you're updating categories form. if you're doing choosing single category add new product to, can simplify this:

class product   belongs_to :category end  class category   has_many :products end  class productscontroller < applicationcontroller   def new     @product = product.new   end    def create     @product = product.new(params[:product])     if @product.save       redirect_to @product, :notice => "successfully created product."     else       render :action => 'new'     end   end end 

if you're assigning product 1 category, don't need many-to-many relationship between them.

as view:

= form_for @product |f|   = f.text_field :name   = f.collection_select :category_id, category.all, :id, :name, :prompt => true