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删除Rails模型中的关系

activerecord ruby ruby-on-rails

Grey · 技术社区 · 6 年前

我使用的是Rails 5,我有一个ActiveRecord“报告”,它有一对多的关系,叫做剂量。

如果是新报告:

report.dosages.size   # Returns 0
report.dosages.build
report.dosages.size   # 1, correct
report.dosages.first.destroy
report.dosages.size  # Still 1 !

我知道它将剂量设置为“销毁”,但实际上可以将其从关系列表中删除? (我以后才能保存DB上的报告)

2 回复 | 直到 6 年前

Alex.U 6 年前

tl;dr:这将满足您的需要:

report = Report.first
dosage = report.dosages.build
report.dosages.size // 1
to_delete_dosage = report.dosages.first
report.dosages.delete(to_delete_dosage)
report.dosages.size // 0

有关详细信息,请查看文档 ActiveRecord_Associations_CollectionProxy::delete

第一, ActiveRecord::Relation#build 是的别名 ActiveRecord::Relation#new :

可以将新对象实例化为空对象(不传递构造参数)或具有属性但尚未保存的预设对象(传递具有匹配关联表列名的键名的哈希)。在这两种情况下,有效的属性键都是由关联表的列名决定的,因此不能有不属于表列的属性。

如果希望删除第一个元素,则必须将其持久化到数据库中,否则Rails将完全按照您的经验进行操作,请设置destroyed标志。

尝试以下顺序:

report = Report.first
dosage = report.dosages.build
dosage.save
report.dosages.size   # 1
report.dosages.first.destroy
report.dosages.size  # 0

我设置了与您的问题等效的输出:

您的方案:

2.4.0 :007 > r.dosages.build
 => #<Dosage id: nil, title: nil, created_at: nil, updated_at: nil, report_id: 1>
2.4.0 :008 > r.dosages.size
 => 1
2.4.0 :009 > r.dosages.first.destroy
   (0.1ms)  begin transaction
   (0.0ms)  commit transaction
 => #<Dosage id: nil, title: nil, created_at: nil, updated_at: nil, report_id: 1>
2.4.0 :010 > r.dosages.size
 => 1

我的建议:

2.4.0 :005 > report = Report.first
  Report Load (0.1ms)  SELECT  "reports".* FROM "reports" ORDER BY "reports"."id" ASC LIMIT ?  [["LIMIT", 1]]
 => #<Report id: 1, title: nil, date: nil, created_at: "2018-07-03 14:00:08", updated_at: "2018-07-03 14:00:08">
2.4.0 :006 > report.dosages.count
   (0.1ms)  SELECT COUNT(*) FROM "dosages" WHERE "dosages"."report_id" = ?  [["report_id", 1]]
 => 0
2.4.0 :007 > dosage = report.dosages.build
 => #<Dosage id: nil, title: nil, created_at: nil, updated_at: nil, report_id: 1>
2.4.0 :008 > dosage.save
   (0.1ms)  begin transaction
  SQL (0.7ms)  INSERT INTO "dosages" ("created_at", "updated_at", "report_id") VALUES (?, ?, ?)  [["created_at", "2018-07-03 14:06:08.709323"], ["updated_at", "2018-07-03 14:06:08.709323"], ["report_id", 1]]
   (0.9ms)  commit transaction
 => true
2.4.0 :009 > report.dosages.size
   (0.2ms)  SELECT COUNT(*) FROM "dosages" WHERE "dosages"."report_id" = ?  [["report_id", 1]]
 => 1
2.4.0 :010 > report.dosages.first.destroy
  Dosage Load (0.2ms)  SELECT  "dosages".* FROM "dosages" WHERE "dosages"."report_id" = ? ORDER BY "dosages"."id" ASC LIMIT ?  [["report_id", 1], ["LIMIT", 1]]
   (0.0ms)  begin transaction
  SQL (0.3ms)  DELETE FROM "dosages" WHERE "dosages"."id" = ?  [["id", 1]]
   (0.8ms)  commit transaction
 => #<Dosage id: 1, title: nil, created_at: "2018-07-03 14:06:08", updated_at: "2018-07-03 14:06:08", report_id: 1>
2.4.0 :011 > report.dosages.size
   (0.2ms)  SELECT COUNT(*) FROM "dosages" WHERE "dosages"."report_id" = ?  [["report_id", 1]]
 => 0

我相信这就是你需要的。关系设置如下:

dosage.rb
class Dosage < ApplicationRecord
  belongs_to :report
end

report.rb
class Report < ApplicationRecord
  has_many :dosages
end

以及以下迁移:

class ManyDosagesToReport < ActiveRecord::Migration[5.0]
  def change
    add_column :dosages, :report_id, :integer
  end
end

事实上,如果你试图坚持 dosage 对象是您构建的,但没有坚持,Rails会抱怨:

RuntimeError: can't modify frozen Hash

由于 destroyed 属性。

Kenigbolo 6 年前

如果我理解正确,您希望从数据库中删除剂量,因此将剂量呈现为 deleted 但是否仍要保留报表对象?如果这是您的用例,那么首先您应该知道报告将保持不变,不管删除的剂量是多少,因为它是1-*(一对多关系),但是您需要查看 delete 和 destroy StackOverflow社区在这里帮助回答 Difference between Destroy and Delete

作为奖励,一定要看看舍邦!Ruby中的方法以及何时使用它,例如 delete! destroy!