外键class_name

您只是在定义错误的关系：

belongs_to :user, foreign_key: 'white_player_id', class_name: 'User'
belongs_to :user, foreign_key: 'black_player_id', class_name: 'User'

game.user在这种情况下将指向white_player_id和black_player_id。

belongs_to :white_player, foreign_key: 'white_player_id', class_name: 'User'
belongs_to :black_player, foreign_key: 'black_player_id', class_name: 'User'

class_name 是相关对象的类。

foreign_key引用模型类的表，您在定义关系时要在其中定义belongs_to关系。

class Game < ActiveRecord::Base
  belongs_to :white_player, foreign_key: 'white_player_id', class_name: 'User'
  # foreign_key is game.white_player_id
end

因此，当我们执行game.white_playerActive Record时，它会寻找：

User.find(game.white_player_id)

添加：

在您的第二个示例foreign_key中，has_many引用了相关表。

belongs_to :parent, :foreign_key => 'parent_id' class_name: 'Folder'
has_many :folders, :foreign_key => 'parent_id', class_name: 'Folder'

而且您不需要显式指定外键和类名：

class Folder < ActiveRecord::Base
  # ActiveRecord will infer that the class name is Folder
  has_many :folders, foreign_key: 'parent_id'
  # Rails will infer that the foreign_key is parent_id
  belongs_to :parent, class_name: 'Folder'
end

如您所见，ActiveRecord是一种智能cookie，可以推断类名和外键。

这是has_manyand的一个更易于解释的示例foreign_keys：

class User < ActiveRecord::Base
  has_many :unread_messages, -> { where read: false }, 
           foreign_key: 'recipient_id', # refers to messages.recipient_id
           class_name: 'Message'
end 

user.unread_messages 将查询表消息：

SELECT "messages".* FROM "messages" WHERE "messages"."recipient_id" # ...