Q:

Creating an object that belong to another - Understand Rails Associations

I have an object called Tasklist that has many Tasks. Simple enough. But Tasklist belongs to User. I'm trying to create a new task from the tasklists show page, and I'm having a ton of trouble figuring out how to do that. I'll post code and explain my problem further. Any help would be great! Thank you.
Tasklists Controller
def show
  @tasklist = current_user.tasklists.find(params[:id])
  @task = @tasklist.tasks.new
end 

Tasklists Show Page
<h1>page<%= @tasklist.name %></h1>

<%= form_for @task do |form| %>

  <%= form.label :description %>
  <%= form.text_field :description %>

  <%= form.submit %>

<% end %>

Tasks Controller
  def create
    current_user.tasklists.tasks.create(task_params)
  end

  private

  def task_params
    params.require(:task).permit(:description)
  end

Here is the error after I try to create the object from the show page.
## ERROR
NoMethodError in TasksController#create
undefined method `tasks' for #<Tasklist::ActiveRecord_Associations_CollectionProxy:0x007faa4263b520>

def create
  current_user.tasklists.tasks.create(task_params)
end

A:

User tasklists returns a collection of records and you are trying to call the instance method #tasks on it. 
Just think about it - how should ActiveRecord know which task_list in the collection you re trying to add a task to?
First you need to untangle your associations:
class User < ActiveRecord::Base
  has_many :task_lists
  has_many :tasks, through: :task_lists
end

class TaskList < ActiveRecord::Base
  belongs_to :user
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :task_list
  # dont use belongs_to as Rails will not keep the relation up 
  # to date!
  has_one :user, through: :task_list
end

Basically we are setting up an indirect relationship between User and Task and ensuring the actual foreign key columns are not duplicated. 
What you want here is a nested resource:
resources :task_lists, shallow: true do
  resources :tasks
end

This will give you these routes among others (run rake routes to see them all) 
            Prefix Verb   URI Pattern                                   Controller#Action
   task_list_tasks GET    /task_lists/:task_list_id/tasks(.:format)     tasks#index
                   POST   /task_lists/:task_list_id/tasks(.:format)     tasks#create
new_task_list_task GET    /task_lists/:task_list_id/tasks/new(.:format) tasks#new

This means we can fetch the task list from params[:task_list_id].
class TasksController < ApplicationController
  before_action :set_task_list, only: [:new, :create, :index]

  # POST /task_lists/:task_list_id/tasks
  def create
    @task = @task_list.tasks.new(task_list_params)
    if @task.save
      redirect_to @task_list, success: 'Task created.'
    else
      render 'tasks/show'
    end
  end

  private 

  def set_task_list
    @task_list = TaskList.includes(:tasks)
                         .where(user: current_user)
                         .find(params[:task_list_id])
  end

  def task_list_params
    params.require(:task).permit(:description)
  end
end

Since we have has_one :user, through: :task_list we don't have to worry about the current_user when creating a task. We just add one to the task_list.
Now lets update the form so that it points to the right resource:
<h1>page<%= @tasklist.name %></h1>

<%= form_for[@tasklist, @task] do |form| %>

  <%= form.label :description %>
  <%= form.text_field :description %>

  <%= form.submit %>

<% end %>

