通过Polymorphic association rails创建对象

Question

I need (or I think) implement polymorphic association in my model but I have something wrong. Let see my situation, it's a simple question/answers system, and the logic is the following: - a question can be ansewered by N answers. - An answer can be only a "text" XOR (one or other, not both) a "picture".

Migrations:

class CreateAnswers < ActiveRecord::Migration
    def change
        create_table :answers do |t|
            t.integer :question_id
            t.references :answerable, :polymorphic => true
            t.timestamps
        end
    end
end

class CreateAnswerTexts < ActiveRecord::Migration
    def change
        create_table :answer_texts do |t|
            t.text :content

            t.timestamps
        end
    end
end

class CreateAnswerPictures < ActiveRecord::Migration
    def change
        create_table :answer_pictures do |t|
            t.string :content

            t.timestamps
        end
    end
end

Models * answer.rb *

class Answer < ActiveRecord::Base
    belongs_to :user_id
    belongs_to :question_id
    belongs_to :answerable, :polymorphic => true

    attr_accessible :answerable_type
end

answer_text.rb

class AnswerText < ActiveRecord::Base
    TYPE = "text"

    has_one :answer, :as => :answerable

    attr_accessible :content
end

answer_picture.rb

class AnswerPicture < ActiveRecord::Base
    TYPE = "picture"

    has_one :answer, :as => :answerable

    attr_accessible :content
end

Controller answers_controller.rb:

...
def create
    post = params[:answer]
    create_answerable(post[:answerable_type], post[:answerable])
    @answer = @answerable.answer.new()
end

private
def create_answerable(type, content)
    @answerable = ('Answer' + type.capitalize).classify.constantize.new(:content => content)
    @answerable.save
end
...

And view form (Only have these fields):

...
<div class="field">
<%= f.label :answerable_type %><br />
<%= select("answer", "answerable_type", Answer::Types, {:include_blank => true}) %>
</div>
<div class="field">
<%= f.label :answerable %><br />
<%= f.text_field :answerable %>
</div>
...

So, the problem is when I submit form I get this error:

undefined method new' for nil:NilClass app/controllers/answers_controller.rb:52:in create'

Answers? :)

Answer 1

on a has_one relationship, you have to use :

@answerable.build_answer

or

@answerable.create_answer

instead of

@answerable.answer.new

see the reference for more info.