has_many build method, Rails - ruby-on-rails

Question

Another newbie question.
The goal: each ingredient can have zero or more unit conversions tied to it. I want to put a link to creating a new unit conversion on the page that shows a specific ingredient. I can't quite get it to work.
Ingredient Model:
class Ingredient < ActiveRecord::Base
belongs_to :unit
has_many :unit_conversion
end
Unit Conversion Model:
class UnitConversion < ActiveRecord::Base
belongs_to :Ingredient
end
Unit Conversion Controller (for new)
def new
#ingredient = Ingredient.all
#unit_conversion = #ingredient.unit_conversions.build(params[:unit_conversion])
if #unit_conversion.save then
redirect_to ingredient_unit_conversion_url(#ingredient, #comment)
else
render :action => "new"
end
end
Relevant Routes:
map.resources :ingredients, :has_many => :unit_conversions
Show Ingredient Link:
<%= link_to 'Add Unit Conversion', new_ingredient_unit_conversion_path(#ingredient) %>
This is the error:
NoMethodError in Unit conversionsController#new
undefined method `unit_conversions' for #<Array:0x3fdf920>
RAILS_ROOT: C:/Users/joan/dh
Application Trace | Framework Trace | Full Trace
C:/Users/joan/dh/app/controllers/unit_conversions_controller.rb:14:in `new'
Help! I'm all mixed up about this.

Answer 1

Unit Conversion Controller for new and create should be:
def new
#ingredient = Ingredient.find(params[:ingredient_id])
#unit_conversion = #ingredient.unit_conversions.build
end
def create
#ingredient = Ingredient.find(params[:ingredient_id])
#unit_conversion = #ingredient.unit_conversions.build(params[:unit_conversion])
if #unit_conversion.save
flash[:notice] = "Successfully created unit conversion."
redirect_to ingredient_unit_conversions_url(#ingredient)
else
render :action => 'new'
end
end
Also, this screencast is a nice resource for nested resources.

Answer 2

has_many :unit_conversion
Should be pluralized since you're calling it with
#unit_conversion = #ingredient.unit_conversions.build
your controller
def new
#ingredient = Ingredient.all
should be calling #new to setup a new Ingredient or #find to grab an existing Ingredient.
#ingredient = Ingredient.new # returns a new Ingredient
or
#ingredient = Ingredient.find(...) # returns an existing Ingredient
Which one you choose is up to your requirements.
Also, this is a typo, right?
belongs_to :Ingredient
You might want to lowercase :ingredient