定义Rails模型的外键关系 [英] Defining foreign key relationships for Rails' models

问题描述

  class Comment< ActiveRecord :: Base 
 belongs_to：post，：class_name => 发布，：foreign_key => post_id，：counter_cache => true 
 belongs_to：author，：class_name => User，：foreign_key => author_id
 end 
  

但是我的CreateComments迁移没有定义数据库 - 级别的外键：

  class CreateComments< ActiveRecord :: Migration 
 def self.up 
 create_table：comments do | t | 
 t.columnpost_id，：integer，：default => 0，：null => false 
 t.columnauthor，：string，：default => ，：limit => 25，：null => false 
 t.columnauthor_email，：string，：default => ，：limit => 50，：null => false 
 t.columncontent，：text，：null => false 
 t.columnstatus，：string，：default => ，：limit => 25，：null => false 
 t.timestamps 
 end 
 end 
 
 def self.down 
 drop_table：comments 
 end 
 end 
  

相反，post_id是一个简单的Integer列。所以，这个外键关系似乎只存在于Rails的头脑中，而不是在数据库级别。

b
$ b

这是正确的吗？

另外，相应的Post模型是否也需要声明其相互的外键关系注释使用：foreign_key属性或可以省略？

  class Post< ActiveRecord :: Base 
 set_table_name（blog_posts）
 belongs_to：author，：class_name => User，：foreign_key => 'author_id'
 has_many：comments，：class_name => 评论，
：foreign_key => 'post_id'，：order => created_at desc，：dependent => ：destroy 
 has_many：categorizations 
 has_many：categories，：through => ：categorizations 
 named_scope：recent，：order => created_at desc，：limit => 5 
 
 end 
  

解决方案

Rails的默认行为是用于在模型上保存外键的列是添加了后缀 _id 的关联的名称。 ：foreign_key 选项可让您直接设置外键的名称。您的发布和评论模型类之间的关联应该如下所示：

  class Post< ActiveRecord :: Base 
 has_many：comments 
 end 
 $ b $ class评论< ActiveRecord :: Base 
 belongs_to：post 
 end 
  

—注意你不需要：class_name =>在您的 Comment 模型中显示Post。 Rails已经有了这些信息。当你需要重写Rails约定时，你应该只指定：class_name 和：foreign_key 。

Rails为您维护外键关系是正确的。如果你想添加外键约束，你可以在数据库层中执行它们。

• 我想你会从阅读中受益 ActiveRecord关联指南。

I have a Comment class with a :foreign_key of post_id in the Post class.

class Comment < ActiveRecord::Base
  belongs_to :post, :class_name => "Post", :foreign_key => "post_id", :counter_cache => true
  belongs_to :author, :class_name => "User", :foreign_key => "author_id"
end

But my CreateComments migration does not define a database-level foreign key:

class CreateComments < ActiveRecord::Migration
  def self.up
    create_table :comments do |t|
      t.column "post_id",       :integer,   :default => 0, :null => false
      t.column "author",        :string,    :default => "",   :limit => 25, :null => false
      t.column "author_email",  :string,    :default => "",   :limit => 50, :null => false
      t.column "content",       :text,      :null => false
      t.column "status",        :string,    :default => "",   :limit => 25, :null => false
      t.timestamps
    end
  end

  def self.down
    drop_table :comments
  end
end

Instead post_id is a simple Integer column.

So, it seems that this foreign key relationship exists only in the mind of Rails, not at the database level.

Is this correct?

Also, is it necessary for the corresponding Post model to also declare its reciprocal foreign key relationship with Comments using the :foreign_key attribute or could that be omitted?

class Post < ActiveRecord::Base
  set_table_name("blog_posts")
  belongs_to :author, :class_name => "User", :foreign_key => 'author_id'
  has_many :comments, :class_name => "Comment",
    :foreign_key => 'post_id', :order => "created_at desc", :dependent => :destroy
  has_many :categorizations
  has_many :categories, :through => :categorizations
  named_scope :recent, :order => "created_at desc", :limit => 5

end

解决方案

The Rails default behaviour is that the column used to hold the foreign key on a model is the name of the association with the suffix _id added. The :foreign_key option lets you set the name of the foreign key directly. The associations between your Post and Comment model classes should look like this:

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

—Note that you don't need :class_name => "Post" in your Comment model. Rails already has that information. You should only be specifying :class_name and :foreign_key when you need to override the Rails' conventions.

You're correct that Rails maintains the foreign key relationships for you. You can enforce them in the database layer if you want by adding foreign key constraints.

• I think you would benefit from reading A Guide to ActiveRecord Associations.

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