Rails ActiveRecorde has_many foreign_key 使用

使用的是rails2.0.2版本，。现有表结构如下:

create table mc$dr_role(
id number primary key,
role varchar2(30),
enabled varchar2(10)
)
create table mc$dr_user_roles(
id number primary key,
user_id number ,
role_id number
)
create table users(
id number primary key,
name varchar2(30) ,
passwd varchar2(30)
)

在model中写法是：

class Role < ActiveRecord::Base
set_table_name "mc$dr_role"
        has_many   :user_roles, :class_name =>"DrUserRole", :foreign_key=>"id",
                            :dependent=> :delete_all  
        has_many  :users, :through => :user_roles
end
class User < ActiveRecord::Base
        has_many   :user_roles, :class_name =>"DrUserRole", :foreign_key=>"id",
        :dependent=> :destroy  
        has_many  :roles, :through => :user_roles
end
class DrUserRole < ActiveRecord::Base
set_table_name "mc$dr_user_roles"
belongs_to :user, :class_name =>"User", :foreign_key=>"user_id"
         belongs_to :role, :class_name =>"Role", :foreign_key=>"role_id"
end

在执行@user.roles << role 报错Cannot associate new records through 'User#user_roles' on '#'. Both records must have an id in order to create the has_many :through record associating them.
我很奇怪关联中“#”是哪里出来的？没弄明白。foreign_key这个配置选项我都已经尝试过好几次变化了，都没成功。。。不知道原因在那里？还是对于这样自定义的表进行关联式有问题的？

正常来说
按照你所需要的建多对多关联表 是这样弄的

ruby generate model role role:string enable:string
ruby generate model user name:string pass:string
ruby generate model UserRole user_id:integer role_id:integer
rake db:migrate

就生成对应的表和字段了
然后是model

class Role < ActiveRecord::Base
        has_many   :user_roles,:dependent=> :destroy
        has_many  :users, :through => :user_roles
end

class User < ActiveRecord::Base
        has_many   :user_roles,:dependent=> :destroy  
        has_many  :roles, :through => :user_roles
end
class DrUserRole < ActiveRecord::Base
belongs_to :user
belongs_to :role
end

over

你的错误应该是指定了错误的外键 导致的..


对于belongs_to :foreign_key是默认的，除非你的，主键不是id，如果你只是修改了table没有改field名的话，考虑去掉:foreign_key。
参考官方API

引用

:foreign_key
    Specify the foreign key used for the association. By default this is guessed to be the name of the association with an "_id" suffix. So a class that defines a belongs_to :person association will use "person_id" as the default :foreign_key. Similarly, belongs_to :favorite_person, :class_name => "Person" will use a foreign key of "favorite_person_id".

那么，楼主的问题在哪呢？

楼主最主要不一样的地方是，打破了一个默认规则，就是不希望，物理表名和Model名一致。

实际，对于这样的需要只需要set_table就够了，也就是model写成这样

class Role < ActiveRecord::Base
set_table_name "mc$dr_role"
        has_many   :dr_user_roles, 
                   :dependent=> :delete_all  
        has_many  :users, :through => :dr_user_roles
end
class User < ActiveRecord::Base
        has_many   :dr_user_roles,
                   :dependent=> :destroy  
        has_many  :roles, :through => :dr_user_roles
end
class DrUserRole < ActiveRecord::Base
set_table_name "mc$dr_user_roles"
         belongs_to :user
         belongs_to :role
end 

那么，最开始说了，什么时候用指定:foreign_key
接着看看，什么时候用:class_name，这个应该是，只有定义的关系和类名不一致的时候。例如：假设楼主，很坚持使用user_roles，就可以按下面写
Ruby代码

 

class Role < ActiveRecord::Base
set_table_name "mc$dr_role"
        has_many   :user_roles,
                   :class_name =>"DrUserRole",  
                   :dependent=> :delete_all  
        has_many  :users, :through => :user_roles
end
class User < ActiveRecord::Base
        has_many   :user_roles,
                   :class_name =>"DrUserRole", 
                   :dependent=> :destroy  
        has_many  :roles, :through => :user_roles
end
class DrUserRole < ActiveRecord::Base
set_table_name "mc$dr_user_roles"
         belongs_to :user
         belongs_to :role
end 

换句话说，这里没有问题，没有不一致的问题。
只是，不应该指定一个，不太合逻辑的foreign_key。也就是说belongs_to的class_name和forein_key可以不去掉，顶多就是重复写了一遍。has_many的foreign_key也可以去掉，因为和默认的外键是一样的，毕竟你的id还是主键。

那么，如果楼主说了，我就偏要写呢，

嗯，也可以，那就像楼上说的，那得把foreign_key写对。
has_many的默认foreign_key是什么呢，那就要看看逻辑啦
你有很多子表，那么外键就是你的主键了，所以应该是，主表名加主id。
或者，也可以参考，官方API

:foreign_key
#    Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_many association will use "person_id" as the default :foreign_key. 

也就是说，楼主的Model写成下面的，也没问题。

class Role < ActiveRecord::Base
set_table_name "mc$dr_role"
        has_many   :user_roles, :class_name =>"DrUserRole", :foreign_key=>"role_id",
                            :dependent=> :delete_all  
        has_many  :users, :through => :user_roles
end
class User < ActiveRecord::Base
        has_many   :user_roles, :class_name =>"DrUserRole", :foreign_key=>"user_id",
        :dependent=> :destroy  
        has_many  :roles, :through => :user_roles
end
class DrUserRole < ActiveRecord::Base
set_table_name "mc$dr_user_roles"
belongs_to :user, :class_name =>"User", :foreign_key=>"user_id"
         belongs_to :role, :class_name =>"Role", :foreign_key=>"role_id"
end 

最后，说说，为什么:foreign_key写成id的问题，@user.roles能找到，但添加会有问题呢？

因为，:foreign_key写成id，@user.roles就变成
DrUserRole.find_by_id(@user.id)
而不是,正确的
DrUserRole.find_by_user_id(@user.id)