Ruby-on-rails Rails has_one 带有类名和外键
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Rails has_one with class name and foreign key
提问by pmerino
I have a Rails model which I use two has_onerelations: requesterand friend. When in the console I use:
我有一个 Rails 模型，我使用两个has_one关系：requester和friend. 在控制台中时，我使用：
f = FriendRequest.all
f[0].requester
I get ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: users.requester_id: SELECT "users".* FROM "users" WHERE "users"."requester_id" = 4 LIMIT 1.
我明白了ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: users.requester_id: SELECT "users".* FROM "users" WHERE "users"."requester_id" = 4 LIMIT 1。
I don't really know how to specify a `has_one' relationship with a class name and a key which specifies the record. This is my model:
我真的不知道如何用类名和指定记录的键来指定“has_one”关系。这是我的模型：
class FriendRequest < ActiveRecord::Base
has_one :requester, :class_name => "User", :foreign_key => "requester_id"
has_one :friend, :class_name => "User", :foreign_key => "friend_id"
end
How could I do it? In a belongs_torelationship I use the same, obviously replacing has_onewith belongs_to. Thanks!
我怎么能做到？在一段belongs_to关系中，我使用相同的，显然替换has_one为belongs_to. 谢谢！
回答by Baldrick
has_one :requester, :class_name => "User", :foreign_key => "requester_id"
This line (from the code that you posted) indicates that the requesteris a User, and the table usersshould contain a column requester_idthat is the foreign key toward friend_requestsrecords. The rails error message states that the column requester_iddoes not exists (you have to create it via a migration).
这一行（来自您发布的代码）表明requesteris a User，并且该表users应包含一列requester_id作为friend_requests记录的外键。rails 错误消息指出该列requester_id不存在（您必须通过迁移创建它）。
In this case, use
在这种情况下，使用
rails generate migration AddRequesterIdToUsers requester_id:integer
It will generate the migration:
它将生成迁移：
class AddRequesterIdToUsers < ActiveRecord::Migration
def change
add_column :users, :requester_id, :integer
end
end
And run them migration with rake db:migrate.
并使用rake db:migrate.
Look at the Rails Relation Guidefor more information on differences between has_oneand belongs_to, and how to use them.
查看Rails Relation Guide以获取有关has_one和之间差异belongs_to以及如何使用它们的更多信息。
