I have the following models:

class Product < ActiveRecord::Base
  belongs_to :brand
  belongs_to :model
  accepts_nested_attributes_for :brand, :model
  ...
end

class Brand < ActiveRecord::Base
  has_many :products
  has_many :models
  ...
end

class Model < ActiveRecord::Base
  has_many :products
  belongs_to :brand 
  accepts_nested_attributes_for :brand
  ...
end

I have a problem to create a new product.

Here is the relevant code in the controller:

class ProductsController < ApplicationController
  ...
  def create
    @product = Product.new(params[:product])
    if @product.save ...     # Here is the error
  end
  ...
end

When user adds a new brand and a new model, params[:product] contains the following:

"brand_attributes"=>{"name"=>"my_new_brand"}
"model_attributes"=>{"model_no"=>"my_new_model"}

and I got the following error:

Mysql2::Error: Column 'brand_id' cannot be null: INSERT INTO `models` ...

because model has a foreign key brand_id which is not set. I can't set it because the brand (like the model) is created on the fly when the product is created. I don't want to create the brand before the product, because then I the product has errors, I will need to delete the created brand.

Then I tried to change params[:product] like this:

"brand_attributes"=>{"name"=>"my_new_brand", 
                     "model_attributes"=>{"model_no"=>"my_new_model"}}

but I end up with this:

unknown attribute: model_attributes

What would be the proper way to handle this ?