Rails 4 Strong Params has_many with JSON - ruby-on-rails

Question

I'm attempting to pass json up on the client side and have rails take care of handling the object creation.
Here are my models:
class Order < ActiveRecord::Base
has_many :order_items, :autosave => true
belongs_to :menu_session
end
class OrderItem < ActiveRecord::Base
belongs_to :order
has_one :menu_item
end
Controller
class OrderController < ApplicationController
#POST /order/create
def create
#order = Order.new(order_params)
#order.save
end
private
def order_params
params.require(:order).permit(:comments, :menu_session_id, :order_items => [:menu_item_id])
end
end
The json data:
{'order': {'comments': 'none', 'menu_session_id': '9', 'order_items':[{'menu_item_id': '5'}, {'menu_item_id': '5'}]}};
The javascript
var data = {};
data.order = {'comments': 'none', 'menu_session_id': '9', 'order_items':[{'menu_item_id': '5'}, {'menu_item_id': '5'}]};
$.post('http://localhost:3000/order/create', orders, function(){}, 'json');
Finally, the error log:
Started POST "/order/create" for 127.0.0.1 at 2013-07-10 22:30:36 -0400
Processing by OrderController#create as JSON
Parameters: {"order"=>{"comments"=>"none", "menu_session_id"=>"9", "order_items"=>{"0"=>{"menu_item_id"=>"5"}, "1"=>{"menu_item_id"=>"5"}}}}
Completed 500 Internal Server Error in 52ms
ActiveRecord::AssociationTypeMismatch (OrderItem(#28109220) expected, got Array(#16050620)):
app/controllers/order_controller.rb:5:in `create'
Clearly, either my json is messed up or the ruby .permit is wrong. However, I've been playing with variations of this for a while now and cannot get it to work. The official documentation doesn't seem to venture into this, and every example I have found here deals with forms.
Anyone have any idea what is going on? I can't be the first to try this approach.
UPDATE:
Worked around it by making the following changes:
class OrderController < ApplicationController
#POST /order/create
def create
#order = Order.new(order_params)
order_items = order_item_params
order_items.each do |item|
#order.order_items << OrderItem.new(menu_item_id: item)
end
#order.save
end
private
def order_params
params.require(:order).permit(:comments, :menu_session_id)
end
def order_item_params
params.require(:order_items)
end
end
json: {"order":{"comments":"none","menu_session_id":"9"},"order_items":["5","5"]}
I don't think this would be the best way to do it, so I'm going to leave the question unanswered for now in hopes there is a best practice.

Answer 1

The workaround is not necessary in this case. ActiveRecord provides an automagic way of creating child elements directly through the params hash. In order to accomplish this, follow the steps bellow:
Configure Nested Attributes in the model
class Order < ActiveRecord::Base
# autosave is already enabled with accepts_nested_attributes_for
has_many :order_items
belongs_to :menu_session
accepts_nested_attributes_for :order_items
end
Include a *_attributes key in your JSON. In your case, change the order_items key to order_items_attributes
{'order': {'comments': 'none', 'menu_session_id': '9', 'order_items_attributes':[{'menu_item_id': '5'}, {'menu_item_id': '5'}]}};
In your controller, make permit accept your new key
def order_params
params.require(:order).permit(:comments, :menu_session_id, :order_items_attributes => [:menu_item_id])
end
There is some more awesomeness possible to accomplish with Nested Attributes. For further information, see ActiveRecord::NestedAttributes at Rails API