Rails 3 - has_and_belongs_to_many

Question

我有2個模特 - 老師和主題 。 想要通過名稱Qualification連接表連接它們。

看起來我做錯了什么：

class Teacher < ActiveRecord::Base
  has_and_belongs_to_many :subjects, :join_table => "Qualification"
end

class Subject < ActiveRecord::Base 
  has_and_belongs_to_many :teachers, :join_table => "Qualification"
end

我的遷移：

class CreateQualificationJoinTable < ActiveRecord::Migration
  def change
    create_table :qualification, :id => false do |t|
      t.integer :subject_id
      t.integer :teacher_id
    end
    add_index :qualification, :subject_id
    add_index :qualification, :teacher_id
  end
end

例如，當我打開rails console並打印時

ruby-1.9.3-head :013 > Qualification

我明白了：

NameError: uninitialized constant Qualification
    from (irb):13
    from /Users/serg/.rvm/gems/ruby-1.9.3-head/gems/railties-3.2.0/lib/rails/commands/console.rb:47:in `start'
    from /Users/serg/.rvm/gems/ruby-1.9.3-head/gems/railties-3.2.0/lib/rails/commands/console.rb:8:in `start'
    from /Users/serg/.rvm/gems/ruby-1.9.3-head/gems/railties-3.2.0/lib/rails/commands.rb:41:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

怎么了？

Answer 1

首先，在遷移中創建表不會定義您的模型。 您必須在app/models/qualification.rb創建一個Qualification模型：

class Qualification < ActiveRecord::Base
  belongs_to :subjects
  belongs_to :teachers
end

其次，如果您使用的是Rails 3，請拋棄has_and_belongs_to_many並使用has_many :through ：

class Teacher < ActiveRecord::Base
  has_many :qualifications
  has_many :subjects, :through => :qualifications
end

class Subject < ActiveRecord::Base 
  has_many :qualifications
  has_many :teachers, :through => :qualifications
end

Answer 2

如果您不打算直接使用連接表，則應該只使用has_and_belongs_to_many 。 在您的情況下，如果您不打算使用Qualification本身但僅使用Teacher和Subject，並使用Active Record執行臟工作，請使用它。 如果您與連接模型有任何關系，請使用has_many：through。

在這里閱讀更多： 選擇has_many：through和has_and_belongs_to_many

Answer 3

在rails 3中，最好的方法是通過遷移進行中間表認證

屬性將類似於此表

subject_id：整數teacher_id：整數

並且像這樣進行資格認證

class Qualification < ActiveRecord::Base
  has_many :subjects
  has_many :teachers
end

然后定義其他兩個模型

 class Teacher < ActiveRecord::Base
   has_many :qualifications
   has_many :subjects, :through => :qualifications
 end

 class Subject < ActiveRecord::Base 
  has_many :qualifications
  has_many :teachers, :through => :qualifications
 end

並仔細閱讀此鏈接

   http://blog.hasmanythrough.com/2007/1/15/basic-rails-association-cardinality