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::Baseset_table_name "mc$dr_role" has_many :user_roles, :class_name =>"DrUserRole", :foreign_key=>"id", :dependent=> :delete_all has_many :users, :through => :user_rolesendclass User < ActiveRecord::Base has_many :user_roles, :class_name =>"DrUserRole", :foreign_key=>"id", :dependent=> :destroy has_many :roles, :through => :user_rolesendclass DrUserRole < ActiveRecord::Baseset_table_name "mc$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:stringruby generate model user name:string pass:stringruby generate model UserRole user_id:integer role_id:integerrake 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_rolesendclass User < ActiveRecord::Base has_many :user_roles,:dependent=> :destroy has_many :roles, :through => :user_rolesendclass DrUserRole < ActiveRecord::Basebelongs_to :userbelongs_to :roleend
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
[quote]: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".[/quote]
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::Baseset_table_name "mc$dr_role" has_many :dr_user_roles, :dependent=> :delete_all has_many :users, :through => :dr_user_rolesendclass User < ActiveRecord::Base has_many :dr_user_roles, :dependent=> :destroy has_many :roles, :through => :dr_user_rolesendclass DrUserRole < ActiveRecord::Baseset_table_name "mc$dr_user_roles" belongs_to :user belongs_to :roleend
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::Baseset_table_name "mc$dr_role" has_many :user_roles, :class_name =>"DrUserRole", :dependent=> :delete_all has_many :users, :through => :user_rolesendclass User < ActiveRecord::Base has_many :user_roles, :class_name =>"DrUserRole", :dependent=> :destroy has_many :roles, :through => :user_rolesendclass DrUserRole < ActiveRecord::Baseset_table_name "mc$dr_user_roles" belongs_to :user belongs_to :roleend
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::Baseset_table_name "mc$dr_role" has_many :user_roles, :class_name =>"DrUserRole", :foreign_key=>"role_id", :dependent=> :delete_all has_many :users, :through => :user_rolesendclass User < ActiveRecord::Base has_many :user_roles, :class_name =>"DrUserRole", :foreign_key=>"user_id", :dependent=> :destroy has_many :roles, :through => :user_rolesendclass DrUserRole < ActiveRecord::Baseset_table_name "mc$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)
