Rails 未加载参考属性

Question

我有两个模型，用户和角色。我添加了一个如下所示的迁移：

class AddRoleReferenceToUsers < ActiveRecord::Migration
   def change
     add_reference :users, :role, index: true
  end
end

现在我的架构如下所示：

create_table "roles", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "users", force: true do |t|
  t.string   "email",                  default: "", null: false
  t.string   "name"
  t.integer  "role_id"
end
add_index "users", ["role_id"], name: "index_users_on_role_id", using: :btree

我的模型是这样的：

class Role < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base
  has_one :role
end

在我的 seed.rb 中，我正在尝试做：

user = User.create(email:"example@example.com" name:"Ben")
<Initialize Role table with some values>
user.role = Role.first

我收到以下错误：

ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'roles.user_id' in 'where    clause': SELECT  `roles`.* FROM `roles`  WHERE `roles`.`user_id` = 1 LIMIT 1
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:301:in `query'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:301:in `block in execute'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_adapter.rb:373:in `block in log'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.1.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_adapter.rb:367:in `log'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:301:in `execute'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/mysql2_adapter.rb:228:in `execute'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/mysql2_adapter.rb:232:in `exec_query'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/mysql2_adapter.rb:240:in `select'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract/database_statements.rb:31:in `select_all'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/connection_adapters/abstract/query_cache.rb:69:in `select_all'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/querying.rb:39:in `find_by_sql'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/relation.rb:603:in `exec_queries'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/association_relation.rb:15:in `exec_queries'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/relation.rb:487:in `load'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/relation.rb:231:in `to_a'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/relation/finder_methods.rb:451:in `find_take'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/relation/finder_methods.rb:98:in `take'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/associations/singular_association.rb:42:in `find_target'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/associations/association.rb:138:in `load_target'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/associations/association.rb:53:in `reload'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/associations/singular_association.rb:9:in `reader'
 /Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/associations/builder/association.rb:110:in `role'
/Users/ryancollins/RubymineProjects/StreamBoard/db/seeds.rb:37:in `<top (required)>'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.1.1/lib/active_support/dependencies.rb:241:in `load'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.1.1/lib/active_support/dependencies.rb:241:in `block in load'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.1.1/lib/active_support/dependencies.rb:232:in `load_dependency'
/Library/Ruby/Gems/2.0.0/gems/activesupport-4.1.1/lib/active_support/dependencies.rb:241:in `load'
/Library/Ruby/Gems/2.0.0/gems/railties-4.1.1/lib/rails/engine.rb:543:in `load_seed'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/tasks/database_tasks.rb:184:in `load_seed'
/Library/Ruby/Gems/2.0.0/gems/activerecord-4.1.1/lib/active_record/railties/databases.rake:173:in `block (2 levels) in <top (required)>'
Tasks: TOP => db:seed
(See full trace by running task with --trace)

当我在模型中明确说明了关系后，为什么还要为角色中的用户寻找外键？

Answer 1

当您在 Rails 中使用 belongs_to 时，您是在说包含 belongs_to 的模型具有另一个对象的外键。在本例中，您已将外键 role_id 放置在 User 模型上。因此，User 模型应包含belongs_to :role，Role 模型应包含has_many :users。

Answer 2

那是因为你使用 has_one 和 belongs_to 相反。

您的模型应如下所示：

class Role < ActiveRecord::Base
  has_one :user
end

class User < ActiveRecord::Base
  belongs_to :role
end