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activerecord [], update, update_attribute, update_column, send

 3 years ago
source link: http://blog.ilibrary.me/2020/03/25/activerecord-update-update-attribute-update-column-send
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Monkey patch

这篇日志起源于rails常见的monkey patch. 在应用中通过object.attr_name=写数据库时遇到一个写数据库字段失败的问题，后来发现字段被同名方法覆盖了(这是为何rails强调TDD的原因),update attr_name: value也有同样的问题。

踩了坑，于是写下日志简单记录一下写数据库字段的问题。

active_record下标访问([], read_attribute, write_attribute, update)支持表字段的访问, 不支持association的直接解析, 下表访问和read_attribute是同一个概念. send是通用的方法调用，包含了read_attribute.

大部分情况下，数据库字段操作应该用[], read_atttribute, write_attribute, 避免用update, 因为monkey_patch的问题, send会不确定性地失效。方法访问也一样.

注意update方法会直接提交修改到数据库，同时update会调用public_send方法，和send方法一样会遇到monkey_patch的问题。

class Board < ApplicationRecord
  belongs_to :user

  belongs_to :p0, class_name: "Dog"
  belongs_to :p1, class_name: "Dog"
  belongs_to :p2, class_name: "Dog"
  belongs_to :p3, class_name: "Dog"
  belongs_to :p4, class_name: "Dog"
  belongs_to :p5, class_name: "Dog"
  belongs_to :p6, class_name: "Dog"
  belongs_to :p7, class_name: "Dog"
  belongs_to :p8, class_name: "Dog"
  belongs_to :p9, class_name: "Dog"
  belongs_to :p10, class_name: "Dog"
  belongs_to :p11, class_name: "Dog"

  def item_at(index) # index = 0
    self["p#{index}"]                   # 不工作, 返回nil
    self["p#{index}_id"]                # 工作, 返回p0.id
    self["p#{index}"] = nil             # 不工作,  unknown attribute
    self["p#{index}_id"] = 1            # 工作, 
                                        # 等同于write_attribute "p#{index}_id", 1
    self.write_attribute "p#{index}_id", 1 #工作
    self.read_attribute "p#{index}"     # 不工作, 返回nil
    self.read_attribute "p#{index}_id"  # 工作, 返回id
    self.update p0: nil                 # update 会调用public_send, 有monkeypatch的问题
    self.send "p#{index}"               # 工作, 返回p0
    self.send "p#{index}_id"            # 工作, 返回id
  end
end

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