在惯用的 ActiveRecord 中查找或创建唯一的关联对象？

Question

如果一个新主题不存在，我如何惯用地创建一个新主题，或者如果该主题存在，我如何创建与会议的关联？

我正在处理一个包含会议和主题的项目。它们都通过一个 ConferenceTopic 对象相互关联。主题的名称是唯一的。

下面是我在会议上创建的名为#find_or_create_topic_with 的自定义方法，它接受名称参数。我这样做是因为当我尝试在会议主题上命名 #find_or_create_by 时，我没有成功为第二个主题命名（下面的 Rails 控制台日志）

The commit for this kludge is on Github.

class Conference < ActiveRecord::Base
  …

  def find_or_create_topic_with(name)
    if topic = Topic.find_by(name: name)
      self.topics.include?(topic) ? topic : self.topics << topic
    else
      self.topics.create(name: name)
    end
  end
end

有关主题的 find_or_create_by 的 Rails 控制台日志

2.1.2 :001 > Conference.last.topics.find_or_create_by(name: "fun")
  Conference Load (1.0ms)  SELECT  "conferences".* FROM "conferences"   ORDER BY "conferences"."id" DESC LIMIT 1
  Topic Load (0.5ms)  SELECT  "topics".* FROM "topics" INNER JOIN "conference_topics" ON "topics"."id" = "conference_topics"."topic_id" WHERE "conference_topics"."conference_id" = $1 AND "topics"."name" = 'fun' LIMIT 1  [["conference_id", 3]]
   (0.1ms)  BEGIN
  Topic Exists (0.4ms)  SELECT  1 AS one FROM "topics"  WHERE "topics"."name" = 'fun' LIMIT 1
  SQL (0.3ms)  INSERT INTO "topics" ("created_at", "name", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["created_at", "2014-08-31 18:57:56.274109"], ["name", "fun"], ["updated_at", "2014-08-31 18:57:56.274109"]]
  SQL (0.5ms)  INSERT INTO "conference_topics" ("conference_id", "created_at", "topic_id", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["conference_id", 3], ["created_at", "2014-08-31 18:57:56.287526"], ["topic_id", 8], ["updated_at", "2014-08-31 18:57:56.287526"]]
   (2.1ms)  COMMIT
 => #<Topic id: 8, name: "fun", created_at: "2014-08-31 18:57:56", updated_at: "2014-08-31 18:57:56">
2.1.2 :002 > Conference.first.topics.find_or_create_by(name: "fun")
  Conference Load (0.6ms)  SELECT  "conferences".* FROM "conferences"   ORDER BY "conferences"."id" ASC LIMIT 1
  Topic Load (0.4ms)  SELECT  "topics".* FROM "topics" INNER JOIN "conference_topics" ON "topics"."id" = "conference_topics"."topic_id" WHERE "conference_topics"."conference_id" = $1 AND "topics"."name" = 'fun' LIMIT 1  [["conference_id", 1]]
   (0.2ms)  BEGIN
  Topic Exists (0.3ms)  SELECT  1 AS one FROM "topics"  WHERE "topics"."name" = 'fun' LIMIT 1
   (0.2ms)  COMMIT
 => #<Topic id: nil, name: "fun", created_at: nil, updated_at: nil>
2.1.2 :003 > Conference.first.topics
  Conference Load (0.5ms)  SELECT  "conferences".* FROM "conferences"   ORDER BY "conferences"."id" ASC LIMIT 1
  Topic Load (0.3ms)  SELECT "topics".* FROM "topics" INNER JOIN "conference_topics" ON "topics"."id" = "conference_topics"."topic_id" WHERE "conference_topics"."conference_id" = $1  [["conference_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Topic id: 7, name: "ruby", created_at: "2014-08-31 18:40:40", updated_at: "2014-08-31 18:40:40">]>

以下是查找主题“有趣”的会议的日志

2.1.2 :015 > Topic.find_by(name: "fun").conferences
  Topic Load (0.5ms)  SELECT  "topics".* FROM "topics"  WHERE "topics"."name" = 'fun' LIMIT 1
  Conference Load (0.3ms)  SELECT "conferences".* FROM "conferences" INNER JOIN "conference_topics" ON "conferences"."id" = "conference_topics"."conference_id" WHERE "conference_topics"."topic_id" = $1  [["topic_id", 8]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Conference id: 3, name: "TooLongDidn'tCareConf", location: "Boston", code_of_conduct: false, childcare: false, last_years_attendance: 0, created_at: "2014-08-31 17:47:35", updated_at: "2014-08-31 17:47:35">]>
2.1.2 :016 > Topic.find_by(name: "fun").conferences.count
  Topic Load (0.5ms)  SELECT  "topics".* FROM "topics"  WHERE "topics"."name" = 'fun' LIMIT 1
   (0.3ms)  SELECT COUNT(*) FROM "conferences" INNER JOIN "conference_topics" ON "conferences"."id" = "conference_topics"."conference_id" WHERE "conference_topics"."topic_id" = $1  [["topic_id", 8]]
 => 1

我希望集合代理同时包含第一次和最后一次会议，并且计数为 2。仅使用 #find_or_create_by 我只能将其与首次创建的会议相关联，而不是找到id 为 nil 的主题。

Answer 1

刚刚阅读了代码，看起来您正在尝试满足三个用例：

• 主题存在且已加入会议。 => 什么都不做。
• 主题存在且未加入会议。 => 将主题添加到会议主题中。
• 主题不存在。 => 创建主题并将其添加到会议主题中。

我认为您的代码很好。真的。没有什么太粗俗的地方，我会立即想重写它。

也就是说，我可能会这样写：

def find_or_create_topic_with(name)
  transaction do
    topic = Topic.where(name: name).first_or_create!
    self.topics.where(topic: topic).first_or_create!
  end
rescue
  # need to handle uniqueness violations, etc.
end

一些补充说明：

• 测试在哪里，兄弟！？
• 为避免竞争条件产生不良数据，您可能希望为 name 的主题创建一个唯一索引（在没有唯一索引的模型中永远不要有一个 validates_uniqueness 来支持它！）以及另一个会议主题的 topic_id 和 Conference_id。

如果有任何不妥之处，请随时在 twitter 上联系我。

Answer 2

它看起来像一个会议 has_many Topics。鉴于此，您应该能够在 .topics 关系上使用 find_or_create_by：

topic = my_conference.topics.find_or_create_by(name: "Name")

# topic will now contain either a newly created Topic that is associated with `my_conference`,
# or will be the existing Topic with name "Name" from `my_conference.topics`.