has_many build method, 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

Answer 1

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

Answer 2

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.