爲什麼在生成關聯中的SQL時，ActiveRecord不使用primary_key和foreign_key列？

Question

我知道我可以使用：finder_sql來手動定義用於獲取關聯記錄的SQL，但我想知道ActiveRecord是否在關聯上使用：primary_key和：foreign_key選項來生成連接SQL。它似乎沒有，但我只是在這裏失去了一些東西？

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更新：爲了更加明確，我的問題是：有一些辦法讓ID列作爲主鍵，但已經AR使用不同的列中加入時，我指定我的關係？

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這裏的例子模型定義和示例控制檯會話...

模型

class Post < ActiveRecord::Base 
    #Columns are: id:integer uuid:string name:string 
    has_many :assignments, :foreign_key => 'post_uuid', :primary_key => 'uuid' 
    has_many :categories, :through => :assignments 
end 

class Category < ActiveRecord::Base 
    #Columns are id:integer uuid:string name:string 
    has_many :assignments, :foreign_key => 'category_uuid', :primary_key => 'uuid' 
end 

class Assignment < ActiveRecord::Base 
    #Columns are post_uuid:string category_uuid:string 
    belongs_to :post, :foreign_key => 'uuid', :primary_key => 'post_uuid' 
    belongs_to :category, :foreign_key => 'uuid', :primary_key => 'category_uuid' 
end 

控制檯會話

#Make a Post 
>> p = Post.create(:uuid => '123', :name => 'The Post') 
    Post Create (0.9ms) INSERT INTO "posts" ("name", "created_at", "uuid", "updated_at") VALUES('The Post', '2010-02-04 00:05:13', '123', '2010-02-04 00:05:13') 
=> #<Post id: 2, uuid: "123", name: "The Post", created_at: "2010-02-04 00:05:13", updated_at: "2010-02-04 00:05:13"> 

#Make a Category 
>> c = Category.create(:uuid => '456', :name => 'The Category') 
    Category Create (0.5ms) INSERT INTO "categories" ("name", "created_at", "uuid", "updated_at") VALUES('The Category', '2010-02-04 00:05:30', '456', '2010-02-04 00:05:30') 
=> #<Category id: 2, name: "The Category", uuid: "456", created_at: "2010-02-04 00:05:30", updated_at: "2010-02-04 00:05:30"> 

#Make an Assignment, associating the post and the category 
>> a = Assignment.create(:post_uuid => p.uuid, :category_uuid => c.uuid) 
    Assignment Create (0.4ms) INSERT INTO "assignments" ("created_at", "updated_at", "post_uuid", "category_uuid") VALUES('2010-02-04 00:05:50', '2010-02-04 00:05:50', '123', '456') 
=> #<Assignment id: 2, post_uuid: "123", category_uuid: "456", created_at: "2010-02-04 00:05:50", updated_at: "2010-02-04 00:05:50"> 

#Try to fetch the category from the post object, which generates the wrong SQL 
>> p.categories 
    Category Load (0.0ms) SQLite3::SQLException: no such column: assignments.uuid: SELECT "categories".* FROM "categories" INNER JOIN "assignments" ON "categories".id = "assignments".uuid WHERE (("assignments".post_uuid = 2)) 
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: assignments.uuid: SELECT "categories".* FROM "categories" INNER JOIN "assignments" ON "categories".id = "assignments".uuid WHERE (("assignments".post_uuid = 2)) 
[...Stack trace edited out...] 

#Also odd: Try to fetch the post from the Association, no SQL generated, we just get nil 
>> a.post 
=> nil 

Answer 1

我能得到您的代碼工作通過改變你的模型定義。

class Post < ActiveRecord::Base 
    set_primary_key :uuid # tell the model about the non standard primary key 
    has_many :assignments, :foreign_key => 'post_uuid' 
    has_many :categories, :through => :assignments 
end 

class Category < ActiveRecord::Base 
set_primary_key :uuid # tell the model about the non standard primary key 
has_many :assignments, :foreign_key => 'category_uuid' 
end 

class Assignment < ActiveRecord::Base 
    belongs_to :post, :foreign_key => 'post_uuid' # notice the value for the foreign_key 
    belongs_to :category, :foreign_key => 'category_uuid' # notice the value for the foreign_key 
end 

除此之外，更改遷移文件提名的類別和型號後，非標主鍵（UUID）。

class CreatePosts < ActiveRecord::Migration 
    def self.up 
    create_table :posts, {:id => false} do |t| 
     t.string :uuid, :primary => true, :limit => 20 
     t.string :name 
     t.timestamps 
    end 
    end 

    def self.down 
    drop_table :posts 
    end 
end 

class CreateCategories < ActiveRecord::Migration 
    def self.up 
    create_table :categories, {:id => false} do |t| 
     t.string :uuid, :primary => true, :limit => 20 
     t.string :name 

     t.timestamps 
    end 
    end 

    def self.down 
    drop_table :categories 
    end 
end 

在調用new/create方法時，不能通過質量分配設置uuid的值。您必須顯式設置primary_key的值。

# this will not work. uuid will be set to 0 
p = Post.create(:uuid => '123', :name => 'The Post') 

# this will work 
p = Post.new(:name => 'The Post') 
p.uuid = '123' 
p.save