Question

我有User和Course之间的关系（典型的注册数据）。 User has_many Course，反之亦然（典型JOIN表格方案）。

我正在尝试将这两个模型之间的先前has_and_belongs_to_many关系迁移到has_many :through关系。我的文件目前看起来像：

class User < ActiveRecord::Base

    has_and_belongs_to_many :courses
end

和

class Course < ActiveRecord::Base

    has_and_belongs_to_many :users
end

和连接两个模型的表名是courses_users。

我现在需要将此关系迁移到has_many :through关联，并将列类型user_id设为string，因为我想使用g_number（字符串）User的属性作为外键。注意：我不关心int和varchar/string之间的效果差异。

简短而简单的问题是我需要users.g_number将enrollments.user_id作为外键引用，两者都是字符串。

我尝试迁移和模型返工是这样的：

class User < ActiveRecord::Base

    has_many :enrollment
    has_many :courses, :through => :enrollment
end

和

class Course < ActiveRecord::Base

    has_many :enrollment
    has_many :users, :through => :enrollment
end

最后

class Enrollment < ActiveRecord::Base

    belongs_to :course
    belongs_to :user
end

然后迁移

class ChangeUserIdJoin < ActiveRecord::Migration
    def self.up
        rename_table :courses_users, :enrollments
    end

    def self.down
        rename_table :enrollments, :courses_users
    end
end

此处一切正常。我可以执行User.courses和Course.users等查询。但现在我想将连接表中user_id列的类型更改为string，以便我可以存储g_number（User上的字符串属性）并加入在而不是id的序列User列。

当我尝试在迁移中将user_id列类型更改为string时：

class ChangeUserIdJoin < ActiveRecord::Migration
    def self.up
        change_column :courses_users, :user_id, :string
        rename_table :courses_users, :enrollments
    end

    def self.down
        rename_table :enrollments, :courses_users
        change_column :courses_users, :user_id, :integer
    end
end

查询Course.users和User.courses开始失败（下面是Rails控制台）。 User.courses返回一个空数组（而在有多个Course个对象之前），并且Course.users因为列类型不匹配而引发异常（这显然是有道理的）：

u = User.take
  User Load (0.9ms)  SELECT  "users".* FROM "users"  LIMIT 1
 => #<User id: 1, username: "director", g_number: "g00000000", password_digest: "$2a$10$dvcOd3rHfbcR1Rn/D6VhsOokj4XiIkQbHxXLYjy5s4f...", created_at: "2016-01-06 01:36:00", updated_at: "2016-01-06 01:36:00", first_name: "Director", last_name: "", role: 0, registered: true> 
2.1.5 :002 > u.courses
  Course Load (0.9ms)  SELECT "courses".* FROM "courses" INNER JOIN "enrollments  ON "courses"."id" = "enrollments"."course_id" WHERE "enrollments"."user_id" = $1  [["user_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy []> 
2.1.5 :003 > c = Course.take
  Course Load (0.7ms)  SELECT  "courses".* FROM "courses"  LIMIT 1
 => #<Course id: 12754, year: 2015, semester: 0, department: 7, course: 101, section: 1, name: "SPA 101 01 - Elementary Spanish I"> 
2.1.5 :004 > c.users
PG::UndefinedFunction: ERROR:  operator does not exist: integer = character varying
LINE 1: ... "users" INNER JOIN "enrollments" ON "users"."id" = "enrollm...

我需要能够加入enrollments.user_id = users.g_number。我需要做什么才能将user_id列更改为string模型/表格中的Enrollment类型，并且仍然可以执行{{1}等活动记录查询}和User.courses？

Answer 1

尝试在注册模型中指定外键和主键，例如

belongs_to :user foreign_key: :user_id, primary_key: :g_number

Rails字符串作为外键

1 个答案: