Creating a category drop down field in Ruby on Rails, using nested forms and has_many :through

Edit: It looks like defining @categories again in the task's create method did the trick of clearing up the error. Still working on actually having the category update when I submit the task, because right now it's ignoring it. Any ideas?

Hi all,

I'm trying to create a drop down list in my first Rails project in order to select from a list of categories for a task. I've used a Categorization model to link the Task model with a Category.

After some effort, I've gotten the drop down to show up properly on the new task form, but when I hit submit, the form gives the following error:

NoMethodError in Tasks#create

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.map

Any help you could provide is really appreciated. Here is the code I'm using (excerpted for brevity)...

The relevant part of my new task form (HAML):

= f.fields_for :categorization do |sub|
    = sub.label :name, 'Category'
    = sub.collection_select(:category_id, @categories, :id, :name, :include_blank => 'Select a Category')

tasks_controller.rb:

def new
    @task = Task.new
    @categories = Category.all
end

def create
    @task = current_user.tasks.build(params[:task])
    if @task.save
        flash[:success] = "Task created!"
        redirect_to root_path
    else
        render 'new'
    end
end

categorization.rb:

class Categorization < ActiveRecord::Base
    belongs_to :task
    belongs_to :category

    validates :task_id, :presence => true
    validates :category_id, :presence => true
end

category.rb:

class Category < ActiveRecord::Base
    has_many :categorizations, :dependent => :destroy
    has_many :tasks, :through => :categorizations

    validates :name, :presence => true
end

task.rb

class Task < ActiveRecord::Base
    attr_accessible :title, :body

    has_many :categorizations, :dependent => :destroy
    has_many :categories, :through => :categorizations

    accepts_nested_attributes_for :categorizations

    validates :title, :presence => true
    validates :body, :presence => 开发者_如何学编程true
end

routes.rb:

resources :tasks

resources :categories do
    member do
        get :tasks
    end
end

Any thoughts? Thank you so much for taking a look, and let me know if you need anything else to help.

Haidn

---

I think you're going to need to add the following line to your categorization.rb:

attr_accessible :category_id

Without that line, you cannot set the category_id in the build method you are using in tasks_controller.rb

Let me know if that fixes it for you!

---

OK, I'm going to try figuring out correctly implementing the save on my own, but I discovered the solution for my original problem, which was declaring @categories again in my Task create method. Hopefully that helps somebody!