Question

Every post has only one category, and i need to access category’s name by something like

p = Post.new
p.category.name = "tech"
p.save

How to do that?

---

   class Category < ActiveRecord::Base
       has_many :posts, :dependent => :destroy

       attr_accessible :name, :image

    end

Post.rb

class Post < ActiveRecord::Base
    belongs_to :category

    attr_accessible :category_id, :name, :text, :lang, :image

end

Schema.rb

create_table "categories", :force => true do |t|
    t.string "name"
    t.string "image"
  end

Answer 1

Your example contains a problem.

p = Post.new
p.category.name = "tech"
p.save

First, you create a new post. Second, you want to assign a name to the category of the post, BUT there is no category assigned. This results in a call like post.nil.name where nil would be the category object, if assigned, which isn’t the case. Since nil has no method name, you get the described error undefined method name for nil class.

To solve this, you first need to assign a category to work on.

p.category = Category.first or p.category_id = 1. After that, p.category will return the category object, therefore p.category.name is valid then because it is called on a category object and not on nil.

tl;dr:

p = Post.new
p.category # => nil
p.category.name # throws an error

p.category = Category.first
p.category # => <#Category ...>
p.category.name # => 'actual name of category'
p.category.name = 'foo' # works now