Rails的belongs_to关系不起作用

Question

Currently I am having issues with the belongs_to relationship between my Page model and my Companies model.

I receive the following error message when I am on the Page show.erb.html and I am accessing @page.company.id: undefined method `id' for nil:NilClass

This error message with and without the attr_accessor method in the companies model.

Page Model:

class Page < ActiveRecord::Base
  belongs_to :company
end

Company Model:

class Company < ActiveRecord::Base
    attr_accessor :id, :name
    has_many :pages

end

Companies Migration:

class CreateCompanies < ActiveRecord::Migration
  def change
    create_table :companies do |t|
      t.string :name

      t.timestamps
    end
  end
end

Pages migration:

class CreatePages < ActiveRecord::Migration
  def change
    create_table :pages do |t|
      t.string :name
      t.references :companies, index: true

      t.timestamps
    end
  end
end

Answer 1

As a page has a single company, your page migration should use:

t.references :company, index: true

(singular) instead of :companies (plural). This should allow rails to populate it properly.

Answer 2

Associations

As Rails is designed to run on a relational database stack, you have to appreciate the role of this structure plays in defining & creating the ActiveRecord associations in your models.

To give you some ideas - ActiveRecord is actually something known as an "ORM" (Object Relationship Mapper) - meaning that it will manage the relationships between "objects" in your app. As Rails, by virtue of being built on top of Ruby, is object orientated , ActiveRecord plays an integral role in managing the relationships between your objects

The problem you're referring to is how to construct these relationships correctly:

---

Models

The answer to your question is:

#app/models/page.rb
Class Page < ActiveRecord::Base
   #fields id | company_id | other | attributes | created_at | updated_at
   belongs_to :company
end

#app/models/company.rb
Class Company < ActiveRecord::Base
   #fields id | title | other | page | attributes | created_at | updated_at
   has_many :pages
end

To demonstrate what the tables should look like, you can use this Rails documentation :

This means that in reference to your question, you can either use Martin 's answer, or just add the company_id column manually (we do that):

class CreatePages < ActiveRecord::Migration
  def change
    create_table :pages do |t|
      t.string :name
      t.string :company_id
      t.timestamps
    end
  end
end

---

Datatables

Some other information information you should know:

1. Rails builds your associations with foreign_keys - whenever you create a "relationship" in your Models, all it does it trigger ActiveRecord to identify the foreign_key for the association:

2. Finally, you also need to appreciate that the identification of "records" in a datatable is determined by the primary key it has. Every time you load an object in Rails, it's actually looking into the database for the corresponding id it has

This means if you wish to identify records in Rails, you need to make sure they have a primary key in the database