How can save array values in one form?

I cannot save array params obj_product[]

Database

|categories|
 |id| |name|
   1    HW
   2    SF
   3    PC

|products|
 |id| |amount|

But after saving the table 'PRODUCTS' should save array params will be like this demo

|products|
 |id| |amount| |category_id|
   1     100          1
   2     200          2
   3     300          3

Controller:

def new
  @categories = Category.all
  @obj_product = Product.new(params[:obj_product])
end

def create
  obj_product.new(params[:obj_product]) 

  if obj_product.save()
    redirect_to :action=>"index"
  else
    redirect_to :action=>"new"
  end
end

View:

<% form_for :obj_product, :url => {:action=>'create'} do |f| %>
   <% @categories.each do |category| %>
     <%= f.text_field :category_id %>
     <%= f.text_field :amount ,:id => "part_#{category.id}", :name => "flow_budget_detail[#{category.id}][amount]" %>
   <% end %>
<$ end %>

Log:

INSERT INTO `flow_budget_details` (`category_id`, `amount`) VALUES( 1, 100)

I should expect this log:

INSERT INTO `flow_budget_details` (`category_id`, `amount`) VALUES( 1, 100)
INSERT INTO `flow_budget_details` (`category_id`, `amount`) VALUES( 2, 200)
INSERT INTO `flow_budget_details` (`category_id`, `amount`) VALUES( 3, 300)

I tried this adding the symbol [] but got error The error occurred while evaluating nil.[]:

in new
  @obj_product = Product.new(params[:obj_product][])

in create
  obj_product.new(params[:obj_product][]) 

I want save my array in this form. Note: The table categories can be more than 3, I wrote 3 in my example.

Please somebody can help me?