Rails 4 Nested Forms, Error: Unpermitted parameter: order_item - ruby-on-rails

Question

My goal is to create a new Order and an associated OrderItem using the same form.
Models
class Order < ActiveRecord::Base
belongs_to :user
has_many :order_items, dependent: :destroy
accepts_nested_attributes_for :order_items
validates_associated :order_items
end
class OrderItem < ActiveRecord::Base
belongs_to :order
default_scope -> { order(created_at: :desc) }
end
View
<% #items.each do |item| %>
<%= form_for(#order) do |f| %>
<%= f.hidden_field :user_id, :value => session[:user_id] %>
<%= f.fields_for :order_items do |oi| %>
<%= oi.hidden_field :product_id, :value => item.id %>
<%= oi.hidden_field :price, :value => item.price %>
<%= oi.number_field :quantity, value: 1, class: 'form-control', min: 1 %>
<% end %>
<%= f.submit "Buy Now", class: "btn btn-primary" %>
<% end %>
Controller
def new
#order = Order.new
#order.order_items.build
end
def create
#order = Order.new(order_params)
if #order.save
redirect_to cart_path
else
redirect_to root_url
end
end
private
def order_params
params.require(:order).permit(:user_id, :custom_item_id, order_items_attributes: [:product_id, :price, :quantity])
end
When submitting the nested form data to the database, the error message Unpermitted parameter: order_item gets returned and only the order data is saved.
Update <-- This is resolved
When I remove the "f." from <%= f.fields_for the form renders correctly and order_params includes the order_items data. This is interesting because the RailsGuide for Form Helpers includes the "f." http://guides.rubyonrails.org/form_helpers.html#nested-forms
Parameter
{"utf8"=>"✓", "authenticity_token"=>"<TOKEN>", "order"=>{"user_id"=>"1", "order_item"=>{"product_id"=>"5", "price"=>"120.0", "quantity"=>"1"}}, "commit"=>"Buy Now"}
The data still does not save to the corresponding models.
Update 2 <-- This is resolved
Updated the createaction in the controller to if #order.save!, below is the error message:
Validation failed: Order items order can't be blank, Order items product can't be blank, Order items quantity can't be blank, Order items price can't be blank, Order items is invalid
I believe that the mistake is in this line of code #order.order_items.build(order_params[:order_items_attributes]) but I am not sure what I need to change.
Update 3 Unpermitted parameter: order_item Error message
From the terminal:
Processing by OrdersController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=><TOKEN>, "order"=> "user_id"=>"1", "order_item"=>{"product_id"=>"5", "price"=>"120.0", quantity"=>"1"}}, "commit"=>"Buy Now"}
User Load (0.5ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 1]]
Unpermitted parameter: order_item
(0.2ms) begin transaction
SQL (1.4ms) INSERT INTO "orders" ("user_id", "created_at", "updated_at") VALUES (?, ?, ?) [["user_id", 1], ["created_at", "2016-03-18 14:58:21.724246"], ["updated_at", "2016-03-18 14:58:21.724246"]]
(14.4ms) commit transaction
The order_itemsdata does not get saved.

Answer 1

First, it seems like your association is incorrect in OrderItem. It should be belongs_to :order instead of belongs_to :order_item_params.
Second, I believe your form should say <%= f.fields_for :order_items do |oi| %> (:order_items not :order_item)
Finally, you should not need to do this in your controller: #order.order_items.build(order_params[:order_items_attributes])
Controller
def create
#order = Order.new(order_params)
if #order.save
redirect_to cart_path
else
redirect_to root#url
end
end
def order_params
params.require(:order).permit(:user_id, :custom_item_id, order_items_attributes: [:product_id, :price, :quantity])
end

Answer 2

The view that rendered the form was not in another controller and not in the orders controller. The show action of tht view did not include an instance variable for order_items. This caused the error messages. Thank you for all of your help. For anyone looking for resources for nested forms below are some helpful resources.
YouTube Video Instructions API dock for fields_for
Go Rails video for Form Nested Attributes and Fields For in Rails
Active Record Nested Attributes
Rails Guide: Active Record Associations