创建没有主键的连接表

I have two tables with a many to many relationship that I am using has_and_belongs_to_many to define the association.

我有两个表与多对多的关系，我使用has_and_belongs_to_many来定义关联。

class Foo < ActiveRecord::Base
  ...
  has_and_belongs_to_many :bar
  ...
end

class Bar < ActiveRecord::Base
  ...
  has_and_belongs_to_many :foo
  ...
end

I also have the class defined to represent the join table

我还定义了表示连接表的类

class BarFoo < ActiveRecord::Base
  ...
  belongs_to :foo
  belongs_to :bar
  ...
end

When I run rake db:seed I get the following error:

当我运行rake db：seed时出现以下错误：

Primary key is not allowed in a has_and_belongs_to_many join table (bar_foo)

If I edit the database and remove the primary key field (ID) from the bar_foo table and then rerun rake db:seed everything works as desired.

如果我编辑数据库并从bar_foo表中删除主键字段（ID），然后重新运行rake db：seed，则所有内容都可以正常工作。

Given the above, what is the preferred means of creating join tables in rails with no primary key?

鉴于上述情况，在没有主键的rails中创建连接表的首选方法是什么？

I also tried using "has_many :bars, :through => :foo" and vise versa but got an error message something like "undefined method 'klass' for nil:NilClass".

我也尝试使用“has_many：bars，：through =>：foo”，反之亦然，但得到的错误信息类似于“未定义的方法'klass'为nil：NilClass”。

3 个解决方案

#1

Yes, primary key is not allowed for has_and_belongs_to_many.

是的，has_and_belongs_to_many不允许使用主键。

You have 2 ways to solve this:

您有两种方法可以解决这个问题：

Remove the primary key on that table. In your migration class:

删除该表上的主键。在您的迁移类中：

create_table :bar_foo, :id => false do |t|
  t.integer :bar_id
  t.integer :foo_id
end

Apart from this, you will have to delete the file bar_foo.rb from app/models and also remove any fixture and test files that might have been generated. A good idea is to call the script/destroy (or rails destroy) to destroy the files and then regenerating the migration.

除此之外，您还必须从app / models中删除文件bar_foo.rb，并删除可能已生成的任何fixture和测试文件。一个好主意是调用脚本/ destroy（或rails destroy）来销毁文件然后重新生成迁移。

Or convert to has_many :through

或者转换为has_many：through

class Foo < ActiveRecord::Base
  ...
  has_many :bar_foos
  has_many :bars, :through => :bar_foos
  ...
end

class Bar < ActiveRecord::Base
  ...
  has_many :bar_foos
  has_many :foos, :through => :bar_foos
  ...
end

class BarFoo < ActiveRecord::Base
  ...
  belongs_to :foo
  belongs_to :bar
  ...
end

#2

If you want to use a HABTM association you shouldn't create a model for it - just a table bars_foos with bar_id and foo_id integer columns.

如果你想使用HABTM关联，你不应该为它创建一个模型 - 只是一个带有bar_id和foo_id整数列的表bars_foos。

If you need the model in between (e.g. if you want to keep track of created_at or some other attributes of the relation) you can add additional model e.g. Barred and then you'd have:

如果您需要介于两者之间的模型（例如，如果您想跟踪created_at或关系的某些其他属性），您可以添加其他模型，例如禁止然后你有：

class Foo < ActiveRecord::Base
  ...
  has_many :bars, :through => :barred
  ...
end

class Bar < ActiveRecord::Base
  ...
  has_many :foos, :through => :barred
  ...
end

class Barred < ActiveRecord::Base
  has_many :bars
  has_many :foos
end

#3

You don't need the model

你不需要这个模型

class BarFoo < ActiveRecord::Base
  ...
  belongs_to :foo
  belongs_to :bar
  ...
end

the has_and_belongs_to_many association will search for a table called bar_foo in your database what you need to do is generate a migration to create this table.

has_and_belongs_to_many关联将在数据库中搜索名为bar_foo的表，您需要做的是生成迁移以创建此表。

rails generate migration add_table_bar_foo_for_association

rails生成迁移add_table_bar_foo_for_association

then you edit your migration and it should look like this

然后你编辑你的迁移，它应该是这样的

class AddTableBarFooForAssociation < ActiveRecord::Migration
  def up
    create_table :bar_foo, :id => false do |t|
        t.references :bar
        t.references :foo
    end
  end

  def down
    drop_table :bar_foo
  end
end

Now your association should work and also if you need the association to have extra attributes on the join you can use the has_many :through way and create a model associated to this.

现在您的关联应该起作用，如果您需要关联在连接上有额外的属性，您可以使用has_many：through方式并创建与此关联的模型。

#1