Question

我需要（或者我认为）在我的模型中实现多态关联，但是我有些不对劲。让我们看看我的情况，这是一个简单的问题/答案系统，逻辑如下： - N个答案可以回答一个问题。 - 答案可能只是“文本”XOR（一个或另一个，而不是两个）“图片”。

迁移

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

模型 * 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

控制器 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
...

并查看表单（仅限这些字段）：

...
<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>
...

所以，问题是当我提交表单时，我收到此错误：

未定义的方法new' for nil:NilClass app/controllers/answers_controller.rb:52:in创建'

答案？：）

Answer 1

在has_one关系中，你必须使用：

@answerable.build_answer

或

@answerable.create_answer

而不是

@answerable.answer.new

请参阅the reference了解详情。

通过Polymorphic association rails创建对象

1 个答案: