belongs_to association error

Question

I have been trying for hours to create Items that belongs_to a Booth. I have declared the following:

class Item < ActiveRecord::Base
  belongs_to :booth 
  belongs_to :category 
end

and

class Booth < ActiveRecord::Base
  belongs_to :user
  has_many :items 

  validates :user_id, presence: true
  validates :name, presence: true, uniqueness: { case_sensitive: false }, length: {maximum: 25}
end

According to the ror documentation, this build method should work since the item belongs_to the booth:

class ItemsController < ApplicationController

 def create
      @item = booth.build_item(item_params)

      if @item.save
        flash[:notice] = "Successfully created product."
        redirect_to @item
      else
        render :action => 'new'
      end
    end

However the error message says that booth is an undefined variable or method. Shouldn't the association have defined the method? I've also tried booth.items.build and many other versions all of which have failed to recognize the association. I think I'm not fundamentally understanding this even after reading the docs many times. Can someone please help? Thanks a lot.

Answer 1

def create
  @item = booth.build_item(item_params)

Assuming this method is in a controller, the problem is that booth isn't defined anywhere. It looks like this is in ItemsController (your question could be a lot more specific in this regard), so what is booth supposed to be?

Your code seems to assume that booth is an instance of Booth, in which case booth.build_item would initialize an Item object associated with it, but it doesn't work because you've never assigned anything to booth.

Where your confusion lies, I think, is in the difference between models and controllers. For example, if you had this in your Item model:

class Item < ActiveRecord::Base
  belongs_to :booth

  def get_booth_name
    booth.present? && booth.name # (1)
  end
end

# (2)
item = Item.find(123)
puts item.booth          # => #<Booth id: 456, name: "My booth!">
puts item.get_booth_name # => My booth!

...both (1) and (2) would work, because in both cases booth is the instance method Item#booth, which is defined by the belongs_to :booth association.

But ItemsController doesn't know much about Booth, and there is no ItemsController#booth method. ItemsController doesn't even know much about Item.

class ItemsController < ActionController::Base
  def some_method
    puts booth.name # (1) => NameError: undefined local variable or method `booth'...
  end

  def another_method
    # (2)
    item = Item.find(123)
    puts item.booth          # => #<Booth id: 456, name: "My booth!">
    puts item.get_booth_name # => My booth!

    # (3)
    booth = Booth.find(456)
    puts booth == item.booth  # => true
  end
end

This time, (1) raises an error because booth isn't a method that exists in ItemsController. (2) will work, however, because the instance of Item has a booth method. (3) also works, because we've retrieved the Booth from the database and assigned it to the variable booth.

(I'm assuming, in both of these examples, that an Item with id 123 exists, and that it's associated with a Booth with id 456.)