Question

我有2个模型 - 教师和主题。想要通过名称资格认证的加入表连接它们。

看起来我做错了什么：

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

我的迁移：

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

当我打开rails console并打印时，例如

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>'

有什么问题？

Answer 1

首先，在迁移中创建表不会定义您的模型。您必须在Qualification中创建app/models/qualification.rb模型：

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

其次，如果您使用的是Rails 3，请抛弃has_and_belongs_to_many并使用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连接表，则应该只使用has_and_belongs_to_many。在您的情况下，如果您不打算使用Qualification本身但仅使用Teacher和Subject，并使用Active Record执行脏工作，请使用它。如果您与连接模型有任何关系，请使用has_many：through。

在此处阅读更多内容：Choosing Between has_many :through and has_and_belongs_to_many

Answer 3

在rails 3中，最好的方法是通过迁移进行中间表资格认证

属性将类似于此表

subject_id：整数 teacher_id：整数

也可以像这样进行资格认证

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

然后定义其他两个模型，如

 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

并仔细阅读此链接

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

Rails 3 - has_and_belongs_to_many

3 个答案: